+1 for this idea, since you can remap the gear up and gear down commands to your joystick. I love driving tanks in Battlefield: Bad Company 2, since I can run and gun at the same time. Shifting wouldn't be hard to model at all, in fact, it could be the throttle itself doing the shifting.
For instance:
6 gears, 180o of throttle pitch
0-11 Brakes
12-21 Reverse
21-30, 1st gear
31-60, 2nd gear
61-90, 3rd gear
91-120, 4th gear
121-150, 5th gear
151-180, 6th gear
Now you can run and gun in peace.
-Penguin