The Velcro/OmniTape option should work for the down quilts. The only thing that might be sticky is if you want to use the supplemental quilt by itself.
The way I'd go about it would be to make the inner quilt a full-length summer weight quilt normally, except for adding Velcro running the length of the quilt on the outer shell.
Then make your supplemental quilt a 2/3 or 3/4 quilt (probably just torso-sized, honestly). Make the inner shell on this one the same size as your outer shell on the full-length and run the opposite type of Velcro (or, with OmniTape, you don't have to worry about this) along the length of the quilt, except on the inner shell this time.
You'd have to have a beefier suspension than the average quilt (that's one of the reasons I'm playing with the idea of Velcro on the hammock body) to handle both quilts' weight during winter, but that should be easy enough to solve. Otherwise, it's just a matter of sizing the quilts properly.
If you want, I can try and draw this out on graph paper. I don't promise that it'll be a good representation, though; my drawings often turn out looking like a fourth-grader's.
Making the quilts modular is going to be a weight penalty, but it should make up for it in ease-of-use. At least, that's my theory.