My friend actually just made his sleeping bag into an underquilt. I haven't talked to him about the results. Only difference is that it's a lightweight sleeping bag and he's using it indoors -- although his basement does get pretty chilly at night.
One thing I was a little unsure of (This goes for ALL underquilts), is how do you keep the UQ/SB nice and snug against the bottom of the hammock? At least without having extra supports via the ridgeline (that's what mine currently has, haha.)