I think there's a recent post on the Crestron Yahoo group addressing this. I'll summarize what I recall:

1) Get a Crestron processor which will receive Crestron touchpanel presses etc.

2) Program an SIO (Serial Input / Output) to send out all needed commands to AMX. The output will not be actual device-specific serial commands, but anything you want. Eg. you could send the ASCII string "WatchDVD" to set the system up for DVD, or "AllPowerOff" to shut down the room or "Blinds50" to lower the shades half-way. Basically you're creating your own serial protocol. If you want to, do it all in Hex to make it more difficult (and for extra credit, figure out how to implement a checksum).

3) Connect Crestron serial port to serial port on AMX processor.

4) Create the AMX equivalent of an SIO which watches for incoming serial text and equates to desired macro / button press.

Oh, here's the thread from Crestron/Yahoo.