Is it possible to make it so that once an OP locks a thread, that they can't just unlock, quickly post, and then lock it again?
Doing that changes a discussion thread into a de-facto
sticky thread for propaganda.
True -- but I don't see anything wrong with that. It is equivalent to an announce-only mailing list, of which there are many in open source.
It's just another flavor of moderation, and the creator of the thread should be able to pick their favorite flavor: open thread, moderated thread (moderator may hide posts), closed thread (only moderator may post), etc.
As long as the moderator is up-front and open about their thread policy. no problem.