I have no idea what I'm doing, but it's out.
The process was: remove fork from the frame, soak in lye overnight to take a lot of aluminum off. Then, while holding the fork in the air (not resting it on anything but your hand) you can hammer on the exposed stem
as hard as you want without damaging the fork. And that got it to move....further down into the fork. So drill a 5/16" hole across the stem, put a 1/4" allen key through, apply some PB blaster (NOW it helps, once the thing is already moving a little) and twist it on out.
Prior to all that, I'd given it a long soak in PB blaster and about four boiling water / cold water cycles previously. Maybe that stuff helped a little but it sure didn't seem like it at the time.
Maybe the lye was unnecessary. Maybe hard hammering alone would have knocked it out. We'll never know.