i've been using a synthetic 20 degree bag as a peapod , and it's just darn uncomfortable ,so i've been thinking about cutting it up , sewing it up and adding some shock cord and runners to my ridgline and using it as an UQ. i'll get some pictures up if all goes well. im sure others have done this before...
I use the setup you describe.
bottom to top: open cell foam
zipped down sleeping bag (30 degree)
So the sleeping bag is sandwiched between the open cell foam and the space blanket.
The down compresses a bit, but still adds a lot of warmth to the system.
I sleep comfortably down into single digits with this setup.
FYI, I use a 15 degree bag inside the hammock.
try it and see how it works for you...