I'm just a noob here, but fwiw, it might very well be that aluminum oxide has formed between the stem and the tube, effectively gluing it into place; if you had a way of immersing the fork, stem, and HT in a tub of household ammonia for, say- maybe 48 hours? It might dissolve enough of the Al2O3 for penetrating oil to do it's thing.

I had a similar problem a few years back with an RB-2 whose PO had managed to ovalize the steerer with a stuck wedge. After soaking the fork in ammonia for two days and slopping it for two more with Liquid Wrench, I screwed an old stem bolt into the wedge from the bottom (from the inside of the fork crown), braced the top of the steerer against a 2 x 4, and gave it four solid whacks with a mallet; popped the wedge right out.