I would have it as a push button on/off switch, as there's no chance I could operate it proportionally while steering and throttling, so it'd have to be operated by a thrust of the hips or somet... Id set it to drop the wing flat to get the best airflow for top speed, and so I can run a mahoooosif wing so the car is litarally forced onto the track for the corners.
Be quite funny to watch the racer on the rostrum, imagine Brian Harris's commentating "Just look how he thrusts his hips down the straight to get the best top speed".
I'll think of some more idea's, for a hip thrusting push button 3rd channel.
Chris