How about a Powerball thingy? Maybe that could help, I don't know. I know they're good for forearm and hand strength and could help the rotation movements too.
The arthritis achy thing is a nightmare
I managed to break a chunk of bone off on the outside of my thumb along with a tendon back in july 2007 and ended up having an op and 8 weeks in plaster. The physio said he couldn't do much more than just get a bit of movement in it so pretty much gave up on it. My biggest problem has been the clutch as my hands are tiny and I have nowhere near full mobility in my thumb now so we've modified the bike with a gadget that makes the clutch lighter and fitted a CRG lever so I can ride a bit. It's not perfect but it's all I can do apart from giving up biking completely which isn't gonna happen. It aches like crazy the minute it gets cold now