I've got a degree in electrical and electronic engineering and I understand the principles of motor control and PWM but, I'm not very familiar with the way radio control systems and servos work though. This web page  describes the basic control protocol for a servo though. Servos expect to see a stream of pulses and the width of the each pulse essentially determines the position the servo adopts:

My I/O board can supply pulses from <1ms to >2ms in width when in servo mode.

Reversing

In order to reverse your robot, it would appear you need a 'Dual-Channel Radio Control Interface' which can handle the differences in the left and right control signals and thus support a reverse mode. Reversing controllers - http://www.4qd.co.uk/fea/rev.html http://www.4qd.co.uk/faq/bmnc4.html#rev

The alternative to this is to use a change over relay to directly reverse the polarity of the power supplied to the motors. The disadvantage with this is that moving motors don't like to be reversed as this jolts the motors and causes some high currents to flow. My main processor can handle this though and ensure that motors are stopped before the polarity is switched. I was planning on writing code to ramp the motor speeds up and down anyway and to ideally include some positional feedback using shaft encoders to track rotational position. http://www.robot-electronics.co.uk/htm/md03tech.htm