Just modulate an RF carrier at one end, send it up the cable, and demodulate at the other end - essentially you need a wireless serial link, but without the radio part.
Its the same as havong 4 radios on diffo frequencies on the bench all communicating to 4 receivers outside somewhere using aerials on each, only instead of transmitting through the air your doing it through a coax.
One thing to consider esp with a diplexer is that your modulated signal will have to have its impedance matched to the diplexer and coax.