Thanks for the ideas, folks.
A bit of a discussion with Artworks here has enlightened me to the fact that new rudder post gland nuts/stuffing boxes are roughly $40US each. That fact pretty much makes these entirely disposable items. I rigged up a bit of a ram over the weekend and dosed the whole thing liberally with PB Blaster, Marvel Mystery Oil, and generous whacks with a 3# sledge. None of that had any effect so far, but I'll give it another week or so. After that, I think I'm just going to cut the rudder post off outide the hull, then use a hole saw to cleanly remove the old assembly in one big, corroded mass. I'll have a little more into a welder, threaded aluminum tube, and a new gland assembly, but will save days' worth of of effort to save what is probably marginal junk anyway. Thoughts?
Here's how things looked last Sunday:
