That's a great idea and one I have trying to achieve. the only bugger is that I don't have any old R/C controllers lying around to do this mod in.
Also the other issue is the value (in Ohms) of the variable resistors in these R/C controllers. Unfortunately I can't just use any variable resistor for reasons explained below.
In the circuit I've posted I noticed that the PWM cycles between 0% and 100% duty cycle over a voltage range of approx 0.8v -> 1.5v. For a really smooth control the variable resistor (potentiometer) needs to be able to adjust over this range from it's centre position to rotated all the way to the left (or right).
I'm currently working out the V=IR calculations I need to select the correct values for R6, R7 & R8(pot).
I've been stuffing around with a "daul-gang" pot which is basically two variable resistors on the same rotating shaft. With each reference wired oppposite to the other. This means that when I rotated the pot to the left to output from the comparator (U1d in the schematic diagram) that "drives" the left channel is active, BUT the right channel (with an extra comparator driving the output for the right) is off. Turning the pot to the right causes the left to become inactive and the right active.
Once I've tested and finalised this I will post the updated schematic.