I recently developed a lateral epicondylitis from rock climbing and probably also from mouse work. That’s why I am also highly interested in alternative solutions (trackballs, etc.). The information provided here is therefore of great value to me, thanks!
One thing I recently stumbled upon (slightly out of topic here) is a Precision Gaze Mouse. It is a new promising approach that combines eye and head tracking to gain fast and also precise control over the mouse pointer. If I wasn’t a Linux user, I would give it a try right away. Currently it is only supported on Windows. The way it is presented, it uses a camera for head tracking and I am not comfortable with such a solution. But the description also mentions IR based head tracking, which requires to wear reflecting stickers, e.g. on a headset.