I have used hot/cold techniques to remove stuck seat posts. After a number of penetrant applications at the insertion point, I heated the frame material with a hair dryer and then wrapped dry ice around the post. I believe I then put the post in a bench vice. inserted a long 2X4 between the seat and head tubes, pushed, and prayed the crack I heard was the post freeing up, and not the frame breaking. It worked for me.
New Britain, CT