I had a Turnigy 9X. Actually, three, before I gave up. They all came broken in one way or another, spent $100 on return shipping alone. Big bummer.
I recently bought a used Futaba FF-7 and installed an FR-sky 8 channel module with telemetry. A total of about $100 for a 7 channel hi quality system. But that's for my radio, this project should preferably work for others too. Like my friend with the RC Big Rid, he has an Futaba 12ch also with a FRSky module in the back.
You did all mention a good point, I hadn't thought the entire process through. How to connect inside the radio?
I guess the most efficient way is to hook the arduino up between the radio motherboard and the TX-module. There is some kind of PWM pulse train going on there, with 8 pulses in a row going to the transmitter. I could out the Arduino in serial there, listen to the signals from the radio, copy the first 6 or 7 pulses and then insert my own 8:th channel.
And as you say, the recieving end should be simple.
All in all, I think a lot of Arduino:ers could utilize this concept for making a cheap but powerful one way link. Even if you do not want to hook it up to a RC handheld transmitter, just get a "DYI module" and use... like this one:
TX: Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking
RX: Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking
The TC basicall uses three wires, Vcc, GND and Signal. That's 5 (or 8 with a different RX) channels for $25. With 5 simultaneous channels you should be able to get a very useable datarate.
Who knows, I bet you'd get satisfactory performance by getting a dual setup and sending data both ways independently.
/Magnus