I like this - it's very neat. This would be great for warmer weather - it would be nice and breezy. Have you thought of making the tarp bigger? You could have it overlap for better rain coverage and with short lengths of cord, tie the sides down.
I'm thinking about it as a purely go to ground dwelling bivy - not as an accessory for the hammock. You could make the bottom out of waterproof material in a bath tub style with a couple feet (height wise) of bug net around and covered with an over lapping tarp that can be hung from a ridge line or poles.. Bet it would be very light weight.
You can make it like the Ted Bug Net and have shock cord on one top side of the bug net for easy entry and exit. The tarp could be sewn to the other side of the bug netting and a couple feet of tarp to stake out on each side. It would be bug proof, be breezy and you would be out of the rain.
I think it would be very easy to make also. Would you mind if I try your idea, scottpash?
I can make a mini mock-up and post the pictures/video on how I visualize it. I'm not very good at explaining descriptions - sorry.
I got in a fight one time with a really big guy, and he said, "I'm going to mop the floor with your face." I said, "You'll be sorry." He said, "Oh, yeah? Why?" I said, "Well, you won't be able to get into the corners very well."