The way that I've gotten totally stuck (rusted/corroded in place) seat posts out is by clamping the top of the seat post, dousing the seat tube with liquid wrench or WD40, and twisting the frame back and forth, using the large size of the frame to get leverage to twist the thing.

It sounds like your seat post is impossible to clamp though because it is probably just a steel or aluminum tube at this point with nothing on the top to grab onto. If you have a drill, though, you may be able to drill all the way through the tube and put a rod through it, which you could clamp on to.

Giving it regular liquid wrench baths before hand may help, especially if you follow them up with some tri flow or other lubricant.