After much experimentation, the data link is working very well by using one servo channel as a "clock" (toggled between high and low to mark new data values, as well as a middle value for synchronization at the start of a message), and a second channel as "data" (set to one of 10 different levels, which I can distinguish fairly easily). A cycle of 300ms seems to be fairly reliable, so I can send one decimal digit 0-9 every 300ms. A simple protocol with a checksum lets me set one of 10 different variables to a value 0-99. The plane then blinks its new tail lights in green for a good receipt, or red for a failed receipt.

I was originally thinking that I would repeat all 4 parameters each time, but at 300ms per digit, it would take a while. Since I really only need to update one thing at a time, I now have the control pendant only sending out data for the currently-selected parameter. That way you can change it, hit the trainer button to send it, and wait for a green flash to confirm it was received.

I tested almost all of the electronics together last night -- the only thing missing from a full mockup would be the motor and ESC. I still want to test how range affects the quality of the data transmission, and there's some cleanup still to do, but in general things are looking good for Wednesday night's photo shoot.