I use a CST L-Trac (trackball) on the left side of my Kinesis Advantage. I'm right handed, but my right hand also had worse problems than my left. I figured if I was going to switch to using my left hand for pointing device, I might as well try switching to a trackball as well.

I'd actually started with a Kensington Expert Mouse (also a trackball, despite the name). I switched to the CST because it has a higher resolution. The one thing I didn't like about the CST was the placement of the middle button, but the CST is easily hackable so I added a "sidecar" with a button for my thumb, and it works really well.

Here's a photo of my setup:

http://i.imgur.com/PYZpWyr.jpg