I’m not sure if the foam idea would very well for me. In order to slide it up into the lumbar area, I’d have to loosen the pack’s waist-belt. If I’m already loosening and tightening straps, it’s only one more step to take the pack off, so I might as well just put the pad in the pack rather than wearing it. And I’m not convinced that having the butt-pad up there in addition to the pack’s lumbar pad wouldn’t make for an uncomfortable mix.

Neat idea, though! I can definitely see its utility if I was only carrying a summit-bag or daypack.

Why bother with putting the pad in the bag? Here’s a better solution. For decades backpackers and climbers in Russia and former Soviet Union used wearable “butt-pads” made of foam polyurethane (basically, the same stuff Thermarests are made from). A 10×15″ piece cut out of an old 1/4″ or so foam pad (not too soft is better, old thermarest works great), puncture two holes on short sides, pass an elastic band through, tie the ends – and this thing is worn on your waist all the time. When you need to sit on cold rock or muddy tree, you just slide it down, and bingo. When you walk, you just slide it up and it acts as an extra lumbar support under the pack. Doesn’t get wet (so no need to keep it in the stuff sack), simple, light, warm and very comfortable. Take it off – and it also works wonders if you need to blow on a campfire ;-). Some people get fancy and add a clipping buckle instead of simple knot – this makes it easier to take the wearable butt-pad off instead of stepping out of it (with the shortage of good buckles in the old days I’ve even seen very creative solutions made from bra locks ;-)

Here are couple real-life pictures:
http://poxod.com/lev/Wyoming97/1_09.jpg
http://poxod.com/lev/Wyoming97/2_01.jpg
http://poxod.com/lev/Wyoming97/5_11.jpg