My last two stuck seatposts had diiferent endings. The first was a steel seatpost in a steel frame. After a week of soaking, heating, and pounding, a drilled it and installed a bolt to pull against. Seatpost broke off. I ended up taking it out in over twenty little pieces with a hammer & chisel. There was a 2" stub left in the frame. The frame was not damaged (much) by my efforts.

The last one was an aluminum post in a steel frame. I soaked it with WD-40 overnight and used my cut-off pickaxe handle to pound it out. Took about 15 minutes. Everything came out OK & undamaged.