The reason for the UQ is to avoid compression of the insulation, a bivy doesn't fix that problem. I've used my military sleep system quite a few times in my hammock with the bivy. In nice weather it doesn't seem to matter. However in cold or stormy weather it can make you a little more comfortable and compensate for lack of a winter tarp and/or not being able to pitch your tarp to block all the wind and such.
I am assuming you are talking about being in the sleeping bag in the hammock. If you are talking about suspending the bivy/bag under as a UQ then pretty much anything that will shape to the hammock nicely will work.