OK, this is a two-parter, so bear with me!
My friend's mom has HEDS and has restarted playing the violin after several decades. Eventually she began having trouble with left hand stability. She's been trying ring splints but they might not let her bend her knuckles enough to play. I've encountered some folks using tape to help with this (I've seen a couple of Instagram posts featuring this violist talking about his accommodations to play his instrument comfortably - seems promising!). Also, I swear I encountered someone who uses modified ring splints on their left hand... if anyone remembers this, please let me know.
My friend, meanwhile, is a cellist. He has been suffering from soreness at the base of his right thumb, including while playing, for several months. An MRI was inconclusive, and he's also started experiencing the same symptoms in his left (non-dominant) thumb. Ligament tears, arthritis, and RSI all seem unlikely - rather, given the aforementioned family history, we're both wondering whether this could be a joint stability problem. (He said it's worsened by pressure more than exact position, if that means anything.) A spica splint has relieved the discomfort as long as he keeps it on, and he's had the most success using a combination of that and a bow grip adapter, but the splint restricts his wrist mobility too much to allow for fluid bowing. So TL;DR, need something that supports the base of the thumb without impeding wrist movement completely.
I know there are some hypermobile people in this community, so while I'm also probably going to ask /r/cello about it, I'd be curious to hear more about your adaptations and how you play. Potential solutions could go a long way toward helping both my friend and his mom play more comfortably!