I always wondered about under covers causing condensation in the under quilt and I think spinntex would not breath that well. The DD material would be great for this due to its breathability, but it does wet through eventually in prolonged rain. I'm not sure that I would like to put something even remotely damp on my UQ. How do folks use a poncho as a tarp and rain gear? How do you set up in the rain? Using the DD poncho as an UC would be easy enough to hang before bed, but it might make those night time calls tough in the rain.
If it is raining just stand under the edge of your tarp and "project" as best you can, you are packing up in the morning so it really doesn't matter.