That's a pretty goot idea. For awhile I was toyin with a similar idea of making a 2 or 3 inch thick quick that was really narrow to add to my UQ in the winter. Zelcro would probibly work better.
I'm trying hard now to make all of my gear work for as many different ways and temps I can. I am more inclined to use a thick UQ yearround underneath. Depending on the temps I can be fine all night with that and nothing overtop. Or I can hang the UQ lower if I am too warm.