Okay, I have eight major commentaries on Daniel, so one could reasonably expect some information in there. Here is what they say about the metal binding the stump of the tree:
Buchanan: "Koch saw the importance of the art drawn in Khorsabad of a sacred tree made with golden leaves and metal bands around the drunk. At the tree's top is a sun disk with wings and an art form of Assurbanipal II within the disk. The disk was a sign of the deity surrounding the king. Mendenhall has shown that the winged sun disk indicated the manifestation, glory, power, and dignity of gods like Horus, Re, Zeus, Ahura Mazda, and Baal Shamem. It often provided a frame for the king himself, as has been done here, to show that the king functioned with the authority of the deity" (p. 117).
Charles: "With a band of iron and brass. The meaning is somewhat obscure. A hope of restoration remained since the stump was left in the ground, but the band of iron and brass seems to be 'a figure of speech for the stern and crushing sentence under which the king is to live' (Bevan), so long as his punishment was to last. The words refer to the king only, as the next verse shows" (p. 92).
Collins: "in a fetter of iron and bronze: This line is extremely problematic. The basic problem arises from the switch from tree imagery to beast imagery. How the fetter of iron and bronze relates to either imagery is unclear. One line of interpretation assumes that the iron and bronze is applied to the stump of the tree (so NSRV). Attempts to provide an explanation for this procedure, however, have been unsatisfactory. The older suggestion of van Lengerke that the reference is to 'the bands of iron put round a tree to prevent it from cracking' has been rejected because there is no evidence of such a custom in the ancient world. There is some evidence of a Mesopotamian custom of putting metal bands on trees. Remnants of a tree with bronze rings or bands were unearthed at Khorsabad, at the entrance to the temple of Shamash. Bands of metal are shown around the trunk of a tree on cylinder seals and slabs from the palace of Ahurnasirpal at Nimrud. The god Asshur hovers over the tree, which has been taken to represent the tree of life. An inscription of Nebuchadnezzar II also refers to coating ceders with bronze, but here the reference is to beams rather than trees. The relevance of this evidence to Daniel, however, is very questionable, especially since Daniel speaks of a root (`qr) rather than a stump. In short, the application of a bond or fetter to the root of a tree that has been cut down is unintelligible. Another line of interpretation assumes that here 'the image passes to the reality: the king is to be bound with metal fetters' [Hartman and DiLella, 176]. This view can claim support from the OG v. 14a: "It was imprisoned and was bound by them with bronze fetters and manacles.' The imagery of the Greek text is also confused: a tree eats grass with the beasts and is then imprisoned...The Greek does not mention a fetter in the initial description of the dream in v. 12, but the introduction of fetters in v. 14 suggests that there was such a reference in the Vorlage...The passage remains obscure, however, and it is likely that something has been lost from the original text".
Goldingay: "Their message describes someone being reduced to animal-like existence; restraint by a metal ring is more likely part of that description -- Jerome compares it with the chaining of madmen -- than an aspect of tree culture, whether designed to keep the tree from disintegrating altogether or to keep it from branching anew" (p. 89).
Gowan: "The dream of a great tree that reached to the heavens, providing shelter and food for all living things, is based on the myth of the cosmic tree, one of the most common of all religious symbols ... A rather full description of the world tree appears in Ezek 31, where it is put to a new use by the prophet ... Ezekiel seems to acknowledge the real power and wealth of the pharaoh, but then he has the cosmic tree cut down, something that has no parallel in Near Eastern mythology ... The band around the stemp is not explained. It has been suggested that it really refers to the binding of the king in his madness, but that is unlikely, since nothing is said of binding him in the fulfillment, when he roams the fields with the animals (vv. 23, 32-33). The many depictions of sacred trees in Assyrian art are highly schematic, so it is hard to tell how they are decorated, but the excavation of the Shamash temple at Khorsabad found a tree trunk with two skillfully embossed bronze bands around it (Loud 1936, 104). This verse may thus be a passing reference to such sacred trees" (pp. 75-76, 78).
Hartman and DiLella: "For the stump of the tree left in the ground (4:12), compare the 'stump' of David's dynastic tree in Isa 11:1, and the 'oak whose stump remains when its leaves have fallen' in Isa 4:13. There is no reason to think that the ancients actually clamped a metal band around the stump of a chopped-down tree, as if to keep it from splitting; here the image passes to the reality: the king is to be bound with metal fetters" (p. 176).
Lacocque: "Nebuchadnezzar's tree is cut down but not eliminated. It serves as a witness to God's universal domination. It remains a stump (v. 12), 'bound with iron and bronze.' A. Bentzen emphasizes that it is not so much a question of guaranteeing the perpetuity of the tottering tree as to promise the prolongation of Nebuchadnezzar's kingship...We mentioned above the importance, in this respect, of the circle of iron or of the bronze chains around the amputated trunk. Nebuchadnezzar's throne is maintained. Fore Bevan (p. 91), it is 'a figure of speech for the stern and crushing sentence under which the king is to lie.' Similarly for Jerome it is a sort of strait jacket, while for Rashi it is what assures the stability of the throne" (p. 78, 80).
Montgomery: "The significance of this metal clamp has given rise to many interpretations, the most common one of which since Jer. is that all madmen are bound, and so, e.g., Heng., Klief., Knab., VLeng. proposed the rationalistic idea that the bond was to keep the tree from splitting, which would be satisfactory if there were evidence that such a practice was followed in ancient arboriculture. Pr. thinks that it figures in general Neb.'s confinement. Others find in it an allegorical mng., e.g., Rosen., Hitz., Keil, Bev. It is best to follow Ra., with Mar., Cha., Torrey, to the effect of the symbolism that Neb. should not be removed, with which cf. v. 23. The text further reads that he should be left in a bond of iron and brass in the grass of the field, which might then mean, exposed to the elements, in parallelism with the following clause, let him be wet with the dew of heaven" (p. 233).
I also have a book by Matthias Henze called The Madness of King Nebuchadnezzar: The Ancient Near Eastern Origins and Early History of Interpretation of Daniel 4 (published by Brill in 1999), and here are his comments on Daniel 4:12:
Henze: "Finally, a third detail in the description of the tree is noteworthy. In both versions of Dan 4, the MT and the Old Greek, we find mention of some sort of fetters wrapped around the root (`qr) of the tree. Unfortunately, the text is problematic at this point, adding to the obscurity of the image. The fetters are part of Nebuchadnezzar's initial dream report only, and they are not mentioned again in Daniel's subsequent interpretation of the dream or in its actual fulfillment at the end of the chapter. In the MT, the fetters are part of the watcher's command to cut down the tree and to scatter the animals who seek shelter beneath it...The text is problematic in part because of the abrupt change of metaphors from tree to Nebuchadnezzar. Whereas v. 11 ends with the tree imagery, 'Let the beasts flee from beneath it, the birds from its branches,' v. 13 begins with a direct report of Nebuchadnezzar's punishment, 'Let his heart be changed.' The shift from one metaphor to the other must therefore occur either in, or right after, v. 12....As the story unfolds, however, we learn of Nebuchadnezzar's actual punishment, 'His body was drenched with the dew of heaven' (v. 30). In light of this later reference, it seems plausible to assume that the prediction in v. 12b, 'let him be drenched with the dew of heaven,' marks the first reference to Nebuchadnezzar himself. The shift in metaphors would then occur in the middle of v. 12, and the command to wrap fetters of iron and bronze around the stump would be the last reference to the tree. In the Old Greek, the fetters are not only mentioned in the initial dream report in v. 12, but occur only a little later in the story, i.e. as part of Nebuchadnezzar's nocturnal vision, just befor ehe awakens, in v. 14. Here, the imagery is even more obscure in that the distinction between the symbol and that which is symbolized is given up entirely...
"The obscurity of the image has invited numerous interpretations. Ancient interpreters display a tendency to interpret this line metaphorically. Jerome, for example, remarks laconically with respect to the band of iron and bronze that 'all madmen are bound with chains.' His reading appears to be influenced by the earlier comments found in Theodoret of Cyrrhus' Commentary on Daniel...According to another explanation, the bands in the biblical vision stand for shackles put on an animal to restrain its movement while it is being led to the pasture. This reading finds support from the Old Greek, which pairs the image of the fetters with the imprisonment motif. It does not work as well with the MT, however, since there the chains are applied to the tree, not an animal. Alternatively, some scholars find in the iron bands a means to protect the root from cracking and to ensure the tree's survival. This explication is more convincing in that it acknowledges that the fetters are wrapped around the tree and not around the animal (i.e., the transformed monarch). There is no evidence, however, that such a practice was ever in use in ancient arboriculture.
"We are on more solid ground when we turn our attention to ancient Mesopotamian art in search for an explanation for the bound tree in Dan 4. The alabaster mural relief from the palace of Assurnasirpal II at Nimrud we have examined above is composed around the 'tree of life.' The trees seen on the relief are bound with heavy (metal?) bands. The tree in the center of the scene on the wall right behind the enthroned monarch has two of these bands, while another tree on the walls of the throne-room has four bands at symmetric intervals up the shaft. Similarly, an impressive row of palm trees, adorned with four rings each, appears on the colour-glazed brick facade of Nebuchadnezzar's throne-room in his palace at Babylon. The frontal facade of the room shows a frieze of lions pacing along on a band decorated with rosettes. Above the lions appears a long row of stylized, slender palm trees, each crowned by three pairs of volute capitals and a sun-like flower. The trees are decorated with four green and yellow rings, in each case wrapped around the tree-trunks. While there can be little doubt that the trees are symbols of life and royal might, the symbolism of the rings is less obvious. Koch has observed that the tree in Dan 4 is stripped of its foliage and branches before it is banded with fetters. Unlike the standardized 'tree of life' which is in full bloom and provides the land with both shelter and nourishment, the tree in Dan 4 is badly pruned and entirely bare, if not ruined before the chains are applied. Koch continues to suggest an intriguing explanation: the only archaeological evidence for such a tree without foliage and branches, and yet adorned with rings, are the staves borne by Nabonidus. The two stele from Harran (H2.A and H2.B) as well as a third stele, also from Teima and dating to the time of Nabonidus (#90837), show the monarch holding an unusually long scepter. The staff is not entirely straight but slightly crooked, like a naturally grown tree. On one stele (#90837) Koch even finds signs of branches that were cut off. What makes these staffs so interesting is the fact that they are adorned with rings, as Gadd explains: "...These rings were no doubt of metal, and so was the ferule which tipped the staff at the bottom..." The parallel between the tree in Dan 4 and the staves carried by Nabonidus is striking indeed. In support of Koch's argument, additional evidence can be drawn from the ornaments which appear on the stelae together with the monarchs" (pp. 83-85).
Henze then goes on to discuss other examples from Assyria. Sidney Smith gave a description of the Assyrian New Year's festival along these lines: "At the New Year festival in Assyria a ceremony took place in the gardens of Nabu's temple, which was probably concerned with a bare tree-trunk. Old fillets of green leaves placed on the trunk were removed, for fresh ones to be placed there, metal bands called 'yokes' were cut off, also perhaps for fresh ones to be put on, and on top of the trunk was set, at least in one case, a golden dish" (Sidney Smith, "Notes on 'The Assyrian Tree,' " Bulletin of the School of Oriental Studies, IV (1926-1928), pp. 69-76. And then there were the metal bands discovered at Dur-Sharrukin, the fortress of Sargon II at Khorsabad, of which Loud wrote: "Unlike the 'tree' at the Sin temple, which was entirely encased in bronze, this 'tree' was adorned with bronze rings or bands extending around its circumference and overlapping at their joints. Within the distance uncovered we find two of them, each 0.70 wide, seperated from each other by a slightly narrower interval" (Gordon Loud, Khorsabad. Part 1 (Oriental Institute Publications, 38), 1936, pp. 104-105. In conclusion, Henze noted that the parallels are imperfect, as in Daniel the tree itself was cut down before the bands or fetters were applied, and the bands represent the tree's fertility in Assyrian ceremony whereas its purpose is unclear in the biblical text. Since the text itself is unclear and no use is made of the fetters in the interpretation, the meaning of the fetters was possibly already lost to the biblical author himself (Henze, p. 90).
I also have Koch's German article, here is the bibliographic reference, BTW:
Koch, K. 1993. "Gottes Herrschaft uber das Reich des Menschen: Daniel 4 im Licht neuer Funde," The Book of Daniel in the Light of New Findings (ed. by A. S. van der Woulde), pp. 104-106
He gives images of the Assurnasirpal II relief and the ones of Nabonidus, I could scan them if you might find them useful.