As a first step in rehabilitating Ol’ Paint, I got her into the workshop today. I removed the pedals because those, at least, are keepers.
Then I removed the wheels and saddle, turned Ol’ Paint upside-down and stuck the seatpost in a very large vise. I tightened it as much as I could. And then I started twisting the frame. (Sorry, I forgot to take a picture at this point.)
Result: by twisting on the frame, first I twisted the vise in its stand, and then I ended up moving the table that the vise is attached to. It’s a heavy table, supporting a drill press as well as the vise. It was all I could do to move the table back into position. The seatpost didn’t budge in the bike frame.
I tried a few times, with the same result. I’ll probably try again later with one or two guys holding the table in place. But if that seatpost won’t budge, then it’s time to give up.