Totally doable (Score 2)

First, it's great that you're trying to adapt to your new situation, and not simply assuming that it is not possible. A dear friend of mine is in a similar situation, and after lots of research on her behalf I landed unequivocally on the following setup:

1. As many buttons as you can carry on your remaining mouse hand; i.e. Razer Naga. I've not used any of the super high button count (>8) Logitechs so I can't comment on them, but the Razer is what you would expect from the company: absolutely top notch. This is all personal preference of course, but obviously you want to load up on as many buttons as you can in your mouse hand.

2. That leaves the all important WASD, plus moar buttons. Without any finger control, grasping, wrist control.. hell, even if you only had an elbow, you can still achieve a level of control at least as good as traditional keyboard WASD by switching to an arcade style joystick:

http://www.xgaming.com/

These things are brilliant, and built like a brick shithouse. You can independently program every directional switch in the stick (i.e. WASD), and every button on the board, simply by plugging in a PS2 keyboard and holding the respective buttons on each device for a few moments. You can program multiple configs, and the controller remembers them while not plugged in. This includes modifier keys like shift, which is great for holding modifiers on your joystick to double the versatility of your mouse buttons. This setup is so versatile, that in my experience it cuts down on the mouse buttons you actually find yourself using (still doesn't hurt to have them though). Absolutely no software required to do this programming, and the joystick itself functions as a usb keyboard; your games won't know the difference at all. There are adapters available for every console as well.

One thing that helped was to find a comfortable spot to put the joystick. Think something chair seat height, as opposed to putting it up on your desk. The unit is tall and will need to sit lower than your keyboard or mouse do. It is also wide, so we ended up orienting it with the joystick at the back and the buttons at the front, rather than traditional joystick on left and buttons on right. It fits comfortably next to you like that (think flight sim setup with a left hand throttle control), and feels way more ergonomic than using the keyboard ever did, disability or no.

My friend has played countless hours of WoW and Borderlands with this setup, and if anything it is outright superior to the keyboard in both instances. The one limitation is that it's not practical to simultaneously use the joystick AND the buttons one-handed (a moot point given the disability), but switching between them is so quick, and the buttons can be moved to your mouse anyway, that this is an easily manageable issue in practice. It's also just plain more enjoyable than a keyboard, really. Playing an MMO or FPS with an arcade joystick really puts a shit eating grin on your face. I plan on getting this exact thing for myself once I have more time to game again, it is that good. And you still have your full keyboard in front of you!