Montbell has adapted a classic tailor's trick by integrating a woven fabric "cut on the bias". By orienting the fabric's warp and weft threads at 45 degrees to most major seam lines the sleeping bag becomes more fluid or elastic in nature.
Found this on a review of one of their bags. Sounds like what I'm looking for
. In addition, it sounds like whatever fabric Montbell uses has some natural stretch, as well. They mentioned "coiled fibers" so perhaps they use a very tight nylon "knit"? Anybody heard of such a thing (sounds comfy, though)?
I don't think I'd need very much stretch (certaintly nothing like spandex or what have you), nylon on a bias combined with proper seams would probably be enough. I'm mainly looking at this as a way to eliminate "air bubbles" between a hanging/sagging UQ and the hammock, and to tension-close the edges of the UQ.
I was going to use extra-tall baffles that would allow some stretch of the inner layer so chambers can collapse a bit, but not enough to affect insulation. That must be what Montbell does, since their exterior is a simple 12D Nylon with DWR.