Wow, that would be a big goof if holosilikon was interpreted by way of an English word. Well, as a loanword, the most obvious source would be Greek of course. The word resembles such compounds as holostèmòn "consisting all of warp-threads", holosphurèlatos "consisting all of hammer-beaten metal", holokhalkos "consisting all of copper", holokhrusos "consisting all of gold", etc. So this appears to be a similar word describing the material from which the object was made. I checked the TLG and did not find holosilikon as an attested word. The Greek word for silk was sèrikos (Latin sericus), as in Revelation 18:12, so holosèrikon would have been the expected form. I'm kind of stumped on what silikon could otherwise signify. The only contender that comes to mind is Latin silicius "of, pertaining to silex", but a cloth made from flint powder doesn't make much sense at all to me, and we would have to presume that it was then loaned into Greek and then into Coptic. The passage is reminiscent of the Testament of Abraham 20:10: "And immediately Michael the archangel stood beside him with multitudes of angels, and they bore his precious soul in their hands in divinely woven linen". The burial story of Adam in the Greek Life of Adam and Eve mentions both linen and silk: "Then he spoke to the archangel Michael, 'Go into Paradise in the third heaven and bring me three cloths of linen and silk (sèrikas, c.e. of surikas)'. And God said to Michael, Gabriel, Uriel, and Raphael, 'Cover Adam's body with the cloths and bring oil from the oil of fragrance and pour it over him' " (40:1-2). So "silk" does make contextual sense, so I wonder if it is possible that sèrikos was not the only form but if other dialectal forms of the loanword existed. But other burial stories emphasize the radiant nature of the clothes and one uses the phrase "garments of light" (Encomium of Eustathius, 128.27-28), so I wonder if the compound is one that refers to the whiteness, brightness, radiance, fiery nature, etc. of the clothes (maybe in that sense clothes made from a fiery material like flint might just barely be intelligible), e.g. hololeukos "entirely white". I notice that Roberts & Donaldson have "a shining wrapper" instead of "a precious silken napkin" (M. R. James) in their translation of the passage in question.
I notice that the book is extant only in fragments in Sahidic but is complete in Bohairic and Arabic. If I were you, I would try to find out what word the Bohairic text has in that sentence, and also what word the Arabic uses. That might help narrow down the meaning. Consider too the possibility of a textual corruption.
ETA. Also the word basilikos "royal, kingly" comes to mind, and royal clothes would also make sense in the context. But holobasilikos sounds utterly bizarre as a neologism (unattested of course) and I can't think of a plausible corruption that would produce holosilikon, unless perhaps a conflation of something like hololeukon basilikon > holosilikon. *shrug*