Bad news. I got a call back from National Roofing Contractors Association, and their rep said that there's no roofing material like that. Such structures aren't designed to take extra weight, anyhow.

Gnu, a down comforter would help, but it might blow away in a high wind.

Maybe I should have a nice, solid door installed between the sun room and the rest of the house, and I'll just close it when it sounds like several snare drummers are practicing in the house.