This has been mentioned some time ago but here it is one more time with some pictures.
A marriage between WBBB and an Exped Wallcreeper (new name is Dreamwalker) sleeping bag and their love child - pea-pod.
Setup is not difficult and it can work without any modifications. I tried this on a 10C day with a bit of wind chill and I was immediately warm inside. I also had a pad, Exped MultiMat, under me.
One of the limitations is that you can not use the bug net. Not that many bugs at 10C, anyway.
Bottom of the bag cinches nicely around the hammock and stays there.
Top of the bag would require an additional cinch-cord to seal the bag well and to keep it close to the body. I've used the cord that tightens the hood. It works but it's a week link.
Here it is with me in it:
The bulge at the top is from my hands still pulling the zipper.
I am quite satisfied that this setup works if only it's to be used out of necessity. When I used the bag inside the hammock with a mat under me, I immediately got a cold but syndrome. I like things that have more then one purpose.