Roger and Steven,
Thank you for the info about tulle. Never heard of the stuff, but will look for it at some fabric outlets. Am hoping to find something around 1/2 oz/sq/yd ( <14 gm/sq/m).
My thought is that baffles of the elastic lingerie, however, will not tear when the bag is stuffed or pushed out of shape, will work better with differential cut, and may be less likely to compress the compartments/insulation when the bag is folded around the body. The conventional alternative is to place pleats in the baffles, which is OK, and would work with the tulle; but I'm hoping to expand the envelope a bit, so to speak.
I understand that the lightest such material is referred to by how "sheer" it is, and plan to search for it using that term. With luck, it will be available as yardgoods. If not, then a bit more of a challenge. Taking the little thousandths of an ounce battery powered scale into the lingerie shop may strike some as a little strange, and I may have some explaining to do. But it should quickly become evident how light the material is.
The big danger, as I understand it, is "runs" in the material, which should not be a problem when it is completely encapsulated within the sleeping bag.
The plan is to use a synthic insulation, like Thinsulate Liteloft, for example, and place batts of it into baffled compartments as is done with down, allowing the synthetic to expand to its full insulative potential, without stitching it to the shell. Also, in this way 6 denier fabric can be used without worrying about the problem of down penetrating the shell.
Will report on the results. Thanks, again.