I'm considering making a unit using a Basic Stamp microcontroller with 3 serial ports to interject the GPS RMC sentences into the data stream between the RH96 and the 396XT. The goal would be to be able to use the RH96 with the GPS as well. Since the data rate for the GPS is at 4800bps and the RH96 runs at 115,200 bps, the GPS sentences would occur at 1/24 the rate of the RH96. The microcontroller would have to queue both the GPS and RH96 serial messages and look for a command response from the radio to know when to send the GPS data (otherwise a data overrun could occur).
Come to think of it, the RH96 has an extra serial port that is only used for upgrades. With a little code change, this could be used as a GPS port and the RH96 could do this 'multiplexing' as well. Upman, are you listening?