In addition to blocking wind chill, there would be some tendency to "trap" some "dead" air, which should add at least a small bit of R value. This might vary with the UQP. For example, the under cover on my HHSS fits pretty snug to the upper edge of the hammock, maybe somewhat hindering air flow and movement, while not as snug to the bottom. My WB poncho/UQP for the WBRR, OTOH, is not snug at any point. Still, to whatever degree the air in the UQ remained still, it might provide a small bit of R value.
As already mentioned, placing unworn insulation between hammock and UQP would significantly increase this air's ability to remain still, adding more R value. I have used this trick for years to boost the HHSS either a little or a whole lot, by adding whatever clothing I am not sleeping in down in the UC under the OCF pad and space blanket. Works like a charm. However, I have little to no experience trying that when using the UC(UQP) alone. But, I'm sure it would help.
Lastly, don't the 2QZQ folks make an UQP designed to support a pad and hold it up against your body when needed? This should add a lot of warmth, and should also help using clothing. And might add a few degrees all by itself, or a lot if being used to cut wind chill.
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