Who else has thumbs that can seize up and become immovable?

First of all, yes you should have a will.
Not for locking thumbs, just on General Principles.

I have several times, had thumbs and fingers lock-up. It just a sign of being over 40. It is no fun when I’m playing my instrument, especially in lessons.

..I’ve never experienced that. Maybe you have Aspergers. :)
Who is Will?

Not my thumbs, but my jaw locks open sometimes. Just for a few seconds. TMJ, I think?

I’m double-jointed in basically all my finger/thumb joints. I have a few random knuckles that get stuck in an awkward position if I move wrong (they bend backwards farther than most people’s knuckles do, and that’s when they might get stuck). It can be painful if not quickly assisted by my other hand to get back to a regular position.

Welcome to the weirdness club ;)

It’s called “trigger finger.” If it gets bad or painfulhave an orthopedic surgeon give you a cortisone shot.
My husband has this in several fingers. Our GP gave the first shot and didn’t do that great. We have a friend who is a plastic surgeon who nailed it and it was better for several months.
If its real bad there is a surgery for it but that’s a last resort which could end up making things worse.

well, it’s just my right thumb and there is no discomfort whatsoever. It hasn’t happened since I posted the question . I think weird things that happen to my right hand have to do with using my mouse. It is just overused.