Here's the data-uplink device in progress:

http://imgur.com/U0XDVCv

So far I have it sending a 6-channel PPM signal via trainer cable to my TX, and successfully driving a servo with the resulting RX signal. Next step is to plug the RX back into the Teensy and add some debugging code so I can see how stable the signal is, then figure out how to encode data to send over the RC link. At the moment I'm thinking something like center=sync, high=1, low=0, but I need to do some testing to find out what size pulses I can reliably detect,. The goal is for the upload process to be self-synchronizing, error-detecting, and not take more than about a second, since the plane will be uncontrolled during it! (unless I wire up a nunchuck for flight controls during upload... it could happen!)

It would be awesome to have a full-time data channel available, but I don't see any easy way to do that short of opening up my TX and patching into the switch contacts, so I'll stick with the trainer idea for now.