Cruise Control for most cars consists of a cruise control computer that takes input from steering wheel controls, vehicle speed signals, clutch pedal switches, brake pedal switches, and a throttle position sensor. They generally work by one of two methods, proportional control, where the cruise control system adjusts the throttle proportional to the error, or by PID control, where the system calculates the throttle position by use of a proportional-integral-derivative control where the integral of speed is distance and the derivative of speed is acceleration.
In the above, note that the cruise control does not check the status of the transmission before engaging. This would be fairly difficult in some manual transmissions anyway. All the cruise control cares about is:
So, in theory, if you can reach the minimum speed of the cruise control system (usually around 25 m/h or around 40 km/h), and your vehicle has a cruise control system, you should be able to engage it. But since driving in reverse is akin to driving in first gear, sustaining that speed if you can reach it would probably destroy your engine (high RPMs) or transmission, or drive line.
However, if anyone cares to try it, please /msg me with the results and I will be more than happy to update my w/u. (I will not be able to reimburse you for damages).
Some information courtesy of http://www.howstuffworks.com/cruise-control.htm
This was a fun NSR!
TDS brought up an interesting point. If you are going in reverse, your car may think you are traveling at negative speed. So if you were going a net speed of 40 km/h backwards, your car may "see" that as -40 km/h. So then you would need to hack your car so that it would see backwards as forwards. Hmmm...