I have a few bikes I also seem to have no hope removing the stuck posts, I am certain they are steel, however its hard to soak them in oil when they are hollowed out.. I need to find some way to "plug/cork" the seat post so the oil doesnt just leak out when the bike is sitting upside down. Removing stuck neck stems is no big deal as the stem itself blocks any of the oil from leaking out!

All other stuck seat posts Id attack with some oil though at the top, and using a monkey wrench to twist the post to free it. The wrench leaves deep claw marks all over the post, but its worked for me about 4 times out of 7 or so!