I guess that won´t work. (hope I can explain it with my bad english):
At the receiver you have three electrical connections:
+ (normally red)
- (mostly black)
signal (yellow...)
You can´t use the signal cable to control a relay. I guess you´ll have to use a switch that is operated by the receiver.
But: I am not the specialist for electronics, so maybe some others have a better idea.
What about a small Servo that activates a switch? If you have enough space for this components?
...or someting like that:
http://www.cti-modellbau.de/product_...ul-4-fach.html
With this you can control 4 functions with one stick of your transmitter...(I got one in my Deutz Allis)