Thanks, Knothead, for your reply!
The mast is down, and the spreader bracket on the "nut" side of the through bolt is off. But I can't remove the spreader bracket on the "bolt head" side of the through bolt without removing the bolt. And the hole in the mast is just big enough to pass the bolt through-it does not emcompass the outside diameter of the compression tube.
Though I can't move the through bolt in it's longitudinal direction, (?"athwartships"), I can jiggle it around a bit. (It doesn't seem to be fixed to the inside of the mast.) So I'm assuming there is a compression tube in there, the inside diameter of which matches the outside diameter of the through bolt, preventing me from sliding the bolt out, with a length of the compression tube equal to the inside diameter of the mast.
I'm thinking about checking out the hardware/auto parts store for some kind of "screw press", sort of like those used to remove stuck faucet handles, but more stout...
Before you do anything else, you might want to grind the head of the bolt off so that you can remove the tang on that side.
Then in the worst case scenario you can take a dremel tool and using a small cutting bit, and a little patience, carefully enlarge the hole around the bolt on one side or the other until you can slide the whole thing out.
I have to say it's kinda rare to have a compression tube that far up inside the mast that isn't cut to be flush with the outside of the section.
The only way to get it into place is from one end or the other using a piece of small line. A big PITA for no good reason.
I'm not saying yours wasn't done that way, just that I haven't seen it done much.
But not too much surprises me anymore.
I would recommend that you get it down to the bare spar, clean up all the corrosion and then decide how to proceed. You should probably replace the bolt and compression tube anyway so you don't have to worry about damaging them in the process of removal. Just don't damage the mast too much.