the g13 has a decent "nub" you can map to your WASD, and would also have the advantage of doing pre-recorded macros for certain repeat functions, like loadout changes or chat messages. would save a lot of the pecking at the keyboard. I had a friend who had no real hand motor control, she had to use a pencil in her mouth to type, was causing some serious back problems.
http://www.edimensional.com/product_...roducts_id=143
Can't say how useful this would be for you, but it's PC and 360, so it should work no problem, if not, there's software you can use to force it to work with defiance - i use x360ce.
Not the perfect solution, but, in the long run, it might help you out for other games as well.