My thumb is aggravating the hell out of me, and while the wrist compression glove has been helpful for the carpal tunnel, it's not designed to immobilize that opposable digit. While searching for info on thumb splints I did manage to discover medical info on the condition. It turns out that this malady is commonly referred to as "trigger finger" - or in this case, "trigger thumb". Neato.
https://www.mayoclinic.org/diseases-conditions/trigger-finger/symptoms-causes/syc-20365100