At this point, I have all of the seams sewn on my Hedeby-style dress based on Peter Beatson's proposed pattern except for the two seams that attach the rear panel to the rest of the dress. So I carefully unpinned the rear panel on one side, and tried the dress on.
It's clear that there won't be enough room for the dress to go all the way around my body unless I cut a larger rear panel, even with the narrow (about 1/2 inch) seams I'm using. So I will be cutting a new back panel for the dress, which I can do. I will probably make it 45 cm wide; that should be enough to give me a little ease, and possibly justify some darts.
The good news is that the dress fits *very* nicely through the front. It is twisting just a bit along the seams connecting the triangular gores, but I think that pressing the dress with a steam iron will cure that sufficiently.