Excellent idea. I had a similar idea except less work was involved. Perhaps it could be part one of this project.
If you start out with a Turnigy 9x / FlySky / ergle / Radio and use an FrSky module in the transmitter for telemetry.
Use the telemetry connection to go to your external GCS, and use an unused channel in the transmitter to send data to the ardupilotmega.
This would make use of existing GCS solutions for now and replace only the XBee / Zigbee comm modules. That would be enough development work in itself, and provided it works reliably provide a springboard for modding the transmitter as a later project.
It actually makes sense since data is already being sent to the model via the transmitter, and with the telemetry module you already have two-way communication.
Hi... This was posted long ago... Since then a lot of hacks were made for the radio. Just search for earl and 9x radio. He has inplimented ful support for telemetry ect.