If you have an RF-sensing amplifier that switces to TX when it sees RF and switches to the correct low pass filter based on the frequency of the signal coming in (an RM Italy HLA300 amplifier for example), then, as long as you back the output from the X108G down a little, that "should" work. Otherwise you'll need to get into building a microcontroller based interface unit that can get the rigs frequency via the voltage on pin 3 of the accessory socket or perhaps via CIV over the USB Port and PTT from pin 2 off the acccessory socker (the mini-din) as this gives you a signal when PTT is pressed, I'd suggest you isolate this connection with a small relay, not to overload the X108G.
Look at page 11 of your operators manual for details of what is on that ACC socket. (note ACC NOT AUX!).
73 Ed.