Should work fine. You should be able to get 1200/2400 baud for good distances, higher at shorter distances though it wouldn't be as reliable. Seeing as how R/C systems are already set up for FSK you can send serial data directly over it without having to bother with Manchester encoding. If your TX has a 'buddy box' ability you can use that to directly hijack the PPM stream without having to open the internals up. Tapping the receiver isn't overly difficult, inside your receiver one leg of the IC is going to have the entire PPM baseband signal on it, you can feed that directly into an I/O pin. Just add a little bit of error checking/correcting data to the transmitted packet (maybe as simple as sending each byte twice and comparing it) and you should be able to do quiet a bit more than just blink 8 LED's with it.
I have an RC transmitter and receiver I bought with the specific intention of doing just that, but I haven't finished the project yet for lack of space to complete it (Living with my fiance in a small apartment no space for a workbench) The last thing I did was open both tx and rx up and check where to tap the signal, was easy to find on a scope. If you have any more questions or need assistance feel free to add to the thread or send me a private message.