Use an interface similar to the one in the Futaba 3PDF.  The transmitter I
developed for FMA Direct used the same concept (before Futaba shipped it).

It was a PIC-based device, used serial EEPROM for messages, mixing parameters,
and state machine (permitting multi-lingual setup).  I had 20 channels which
were mixed down to 8.  It counted trims as channels at a low level, but the user
never knew the difference.

Andy