IIRC, there are hobby R/C transmitters that take a serial input; they are meant to be hooked up to a PC to allow flight simulator software to control an actual R/C plane, among other things.
That PC interface method is just a application of a feature that many R/C transmitters have, as follows. Most or at least many better quality R/C transmitters are designed and have a rear multi-conductor jack that allows two transmitters to be linked together in a master trainer/student trainee mode such that the student's joy stick are active (and the student's transmitter is the only one outputting an RF signal to the model) unless the trainer flips a switch and the trainer's joy stick commands are the ones routed to the student's transmitter and sent out the airwaves. Makes for the safest method to teach new R/C pilots how to fly their expensive plane.
Anyway this method utilizes a serial PPM data stream that is always used somewhere inside any R/C transmitter and an arduino could certainly emulate this PPM serial stream along with the digital override signal to the R/C transmitter. The main problem implementing this is that you really need the users manual to your specific transmitter which shows that it indeed has this feature and the pin out of it's remote connector.
Lefty