here's something that I thought of last night... I don't really think it'd be possible, and if it were, the programming would be an absolute nightmare but anyway... instead of joystick controls, have the robot navigate by coordinates that correspond to a computer drawing tablet( that could easily be painted or marked to match the field) and the robot would be moving in relation to the stylus.
a step up would to even use a mini-mouse (with my thoughts, wireless would be best) instead of the stylus and then the buttons on the mouse could be used for robot functions, and here's where the wireless would come in handy, to make steering easier, make the robot rotate in conjuction with the mouse (front of mouse = front of robot) obviously, some type of omnidrive system(crab, macadam, or otherwise) would be ideal.
Again, probably would never be possible, and if it were, it'd prob. take years to program, but wouldn't it be neat?!?