I'd also seriously consider the underquilt being separate. I know its a bit more but then the hammock is more versatile. You don't always have the warmth, which is good when its hot out...
There are days up here when I don't want bottom heat, I'm sure the same applies for your home area.
Finally working on the down UQ will be easier without it being attached to the hammock. The UQ is going to be bulky enough to work on as it is, without adding more fabric.
I've been thinking about that, and it's probably the way i'm going to make it. So what is the material of choice for underquilts? DWR for the outside, nanoseeum for baffles, can the inner fabric be DWR as well?