this summer I bought a
Gunnar Rock Hound with a stuck post. I dissolved the post out of the frame with 100% LYE (sodium hydroxide). this works fine if:
1. you're going to paint the frame (there's a bit of heat generated, and it's caustic soda, so i think it trashes the paint. i had pre-trashed the paint with a torch, so not sure how much damage the lye does)
2. the frame is steel (the lye dissolves aluminum, not steel or plastics or rubber gloves)
3. you have a couple of hours to stand around repeatedly pouring lye and water into a frame plugged with something (like balled up latex gloves)
good luck.
Did it last year with my surly after trying every thing else to get the broken and stuck seatpost out of it. Didn't hurt my paint but the paint wasn't pretty to begin with.