The CCP module can be setup to generate a 22kHz squarewave. Use an external filter to turn it into a sine and attenutate it 650mV, then use a rail-to-rail OP-Amp to buffer it and (I'm guessing) AC-couple it onto the coax. Then it's just a matter of turning the CCP module on and off with the correct timings to get the signal out.

I don't know, you may be able to just attenuate and AC-couple the square wave directly onto the cable.

Do you have a PIC already that you wanted to use?