Success! of a sort.
Starting with 45-inch bowes, I managed to make a 30-inch braid of three "bowes," despite the fact that two of the bowes broke, near the end. It's not perfect, but overall it looks good enough to use.
The problem is that the completed braid will not reach from the bottom of the trim I intend to apply around the bust of the dress all the way to the hem; it falls about six to eight inches short.
I may use it anyway. After all, we don't really know how long the braid on the Hedeby dress extended; for all I know, Beatson's suggestion that the braid ran all the way down each side of the dress from top to bottom may be based on nothing more than modern ideas of what looks "right."
I think I'll make another braid (starting with slightly longer bowes) and continue pondering the question for a bit. Since I seem to have learned enough fingerloop technique for my purpose, I don't intend to post again on this subject for now.