Got it! I resorted to sawing, took the top off and cut vertical slots down the sides. It was a LOT of work, started out fast, but the last little bit took forever, on each of the cuts. The thin blades tended to wander at the end of the cuts. I used hacksaw blades to get the cuts started, and once they were deep enough I was able to fit in a bigger, stiffer blade from a 'Diablo' reciprocating saw.

I ended up accidentally taking off the top half or so in sections, they made four neat 'lobes' and when I tried twisting them they simply broke off. But it was actually easier to work on the rest that way. A blow from a 5 lb sledgehammer on a cold chisel placed down the seattube so its corners hit the cuts helped push the remains a bit down, so I knew I was close, and another few saw strokes brought up one section, then the rest.

Patience is key...this was a lot of work. But it would have happened a lot faster if I'd simply gone for the saw as soon as the first bit of brute force didn't do it. Good luck to anyone who finds this thread looking for advice.

BTW once the post was out I could see where the lye/drano had been working. It did eat away some of the aluminum but it would have taken YEARS to do any good.

Oh, and I used 'Freeze Off' as recommended elsewhere. By itself it didn't seem to do any better than 'Liquid Wrench,' PB Blaster, etc, but at the end I was squirting it in there pretty often and it may have helped.