I guess it depends on how bad your joint issues are. Most of the work involved in cutting, drilling, squaring and finishing pen blanks are done by tools with jigs that don't require a lot of pressure from your hands and fingers. Assembly also is done by a press...There can be some torque involved with tightening and loosening chucks to hold drill bits and pen blanks, and the actual turning requires the ability to drip the turning tools securely, but with sharp tools I would say you don't need too tight a grip. But for some this may be quite difficult. Hopefully not for you.
I like to play guitar a little each day - if I can - and my fingers are stiffer the next day from that activity much more so than any day I've spent turning. I like to think both activities are therapeutic! Hope that helps and good luck!
Russ