I don't have RSI, but I do use a Logitech Trackman Marble FX at work and a Trackman Marble at home. I don't like having to move my hand and wrist around to use a mouse.

You might also try (and this will sound utterly strange) turning your mouse upside down and holding it that way, moving the mouse ball with your thumb and operating the buttons with your fingers. It takes practice, but I'm seen it used quite successfully.

If you have an optical mouse or another type without a ball this won't work, of course. And the mouse ball must fit fairly snugly in its little hole so that it will work against the contacts.