I now have 2 steel frames with alu seat tubes stuck in them, Mk1 PA and old steel stumpjumper. Have tried WD40 and reasonable force on both [smacking seat with mallet]. I am happy to loose the seat tubes, so what to try next?
Caustic soda - really nasty stuff, melts alumium quite effectively, doesn't damage steel or ti. I got a bottom bracket with a sheared off collar thing stuck in a steel frame out like this. It's a bit last resort and you'll need to mind your paintwork and your skin, eyes etc, but it does the job.
On the carbon front, the good news is that there's a special penetrating stuff designed to loosen carbon fibre, but the bad news is that I also have a carbon post jammed in a steel frame and it didn't help. Sorry.