So much for posting blog entries to Mastodon. Neither plugin seems to be working, nor does it seem possible to unbork either of them.
The plugin I was using, Mastodon Autopost, just switched to OAuth, which doesn't work, at least not on this instance. The other, Mastodon Share, simply shows the connection as "disconnected" without any way of, well, reconnecting; deleting and reinstalling doesn't fix the problem.
Open source strikes again. Too many possible points of failure. Giving up for now.