Ozark,

You could use a closed cell sit pad & toggle attatch it (see the thread on this) to the 3/4 length Thermarest at night for full length comfort. No?
See the thread "attatching pillow to pad" and in particular the link in Trailrunner's reply. That toggle system is absolutely BRILLIANT! Hee,hee

The toggles (purchased at a fabric store) tethered with tent cord are sewn to your 3/4 length lightweight Thermarest corners.Then holes melted thru duct tape-reinforced corners of the closed cell foam recieve the toggles. This sitpad setup meets the UL backpackers goal of "multiple use".

Eric

P.S I carry a full length UL Thermarest in summer B/C I love it and feel the extra few ounces are my ONE concession to luxury. But for winter I've just converted my regular 3/4 length Thermarest to a "toggle & sitpad" system on top and a full length RidgeRest on bottom.