How about using a propane torch to burn the seatpost? I don't know how hot you'd have to get the seatpost before it burns, but steel can take a lot of heat before it starts changing properties. Do it in a semi-dark area, and if it starts glowing, stop applying heat. It'll mess up your paint, though. I don't know for sure if it would work, but it's an idea.
Do you still have any of the seatpost that you removed? If so, you could find a junk frame and test it out. A propane bottle and screw-on torch head cost less that $15 last I checked.
Definitely avoid breathing the fumes.
Keith