Maybe one could make baffles (or buffer as WV suggested) of some sort in the ends just to keep the air in. One on the inner layer and one on the outer, these would interlock when the ends are tightened with shock cord. It would still leave small gaps, but probably helps to keep the air in. (After writing this I realised I wrote the same thing as WV, so what he said).
ClockWork, sounds like a good idea to shave some weight off, this would still leave the same problem with air. I think the idea using air as an insulation would be more plausible with one piece quilts. Silnylon + seam sealer + down + air gap = ???, there would be condensation problems of sorts.