In the new "Isaiah's Prophecy Light for All Mankind" book 2 from the Society, an explanation of Isaiah 54 is given, starting on page 216. It explains the tent expansion corresponds to Jerusalem's expansion of Jews after the Babylonian exile, with an expanded fulfillment on the Christian Congregation, starting in 33 CE. The "barren women" gives birth Christ at baptism and to a literal number of 144,000, when they become "anointed", so the book claims. Then on pages 218-219, Abraham's family is related to different groups. Isaac "typifies the spiritual Seed of God's woman," which includes primarily Jesus Christ, with his "anointed brothers." Hagar, "pictures earthly Jerusalem" under the Law Covenant, whereas Ishmael, pictures the first-century Jews under the Law Covenant.
So far, so good, however, while a general reference to Galatians 4:21-31 is made, there is no discussion of Galatians 4:27. There it says, "the children of the desolate woman are more numerous than those of her who has the husband." Anointed Christians are then equated to Isaac (vs. 28) and Jews living under the Law are equated to Ishmael (vs. 29). So then, how is it possible for the spiritual Seed of God's woman to be limited to a literal 144,000 when there was most certainly a greater number of Jews, perhaps in the millions, then living when those words of Galatians were written? Isn't it incredibly clear that scripture says, "the children of the desolate woman are MORE NUMEROUS"? Evidently, Paul was making the point that the full number of those spiritual sons was going to increase in size way beyond the number of Jews under the Law.
Interestedly, the Society wiggles-around fully explaining this scripture, because they evidently can't come up with an easy way to twist the scriptures. In fact, there are only 2 Watchtower articles I've found, written in the early 1960's, which make an attempt at explaining it. Don't hold your breath though! These articles raise the question, but never really answer it.