I have had a similar concept in my head for a while now, but I haven't obtained the materials to try it out yet.
My idea is to make a bathtub floor using the very thin plastazote or evazote sheet. Then attach bugnet to three sides, leaving one side unattached. I was thinking of using velcro for the remaining side, or maybe just a portion of it.
I plan on not sewing up the top, but instead just tying the hemmed netting. Then I can still use my groundsheet when bugs aren't an issue. Just tuck the netting around the inside of the bathtub floor. I will probably make something specific for the tying, so attaching to my tarp roof is easy.
I think your ideas sound great, just wanted to throw in some other zany concepts. Mine wouldn't be incredibly light, but serves as a water proof groundsheet, has some thermo-decoupling properties, and should be pretty durable. I have aslo been thinking of gluing momentum to the underside to give it a bit more durability, but this would add to the weight substantially, so haven't decided yet on this one.
Good luck with your design,