Well - I finally got around to testing the idea of a tank built on the Lego R/C car. This has one PWM output for speed, with steering rotating an axle hole on the receiver. In a normal car, this drives a steering mechanism, but I wanted to drive a dual track tank. I built a little test rig with wheels and skids rather than tracks just to see if it would work.
The basic idea is to have an adder/subtracter drive, with the R/C "drive" output going to a motor on the base input and the steering output tied to a Lego polarity switch. The polarity switch is supplied from a battery box and the output is connected to a motor on the added/subtracted input of the differential mechanism.
I used the R/C motors for both driving and steering, so it whips around pretty fast. It isn't pretty, but it gives me enough
confidence to make a tracked base with the same drive, now:
A front view showing the polarity switch:
And a shot of the differential adder/subtracter drive, straight from Doug's page here: