Should have known someone would have tried it
I was meandering around the surplus store yesterday and wondered if anyone had tried a poncho liner as an UQ. Then here it is.
As far as a second hammock holding it up underneath, what about a supremely cheap (and small) mesh hammock? At same surplus store, I saw a couple that weighed pretty much nothing and packed down to about about the size of a baseball. If you want to go with a hammock hanging underneath, I would think that could be an answer. Probably cheaper than even DYI (the one I saw was about $10). As far as drooping, that seems like an issue that could be corrected by possibly clipping the sides to the hammock, perhaps? For compression, it shouldn't be any different than any other UQ system or a true double layer hammock.