I'd cut the trace from the modulator section feeding the PLL so the TX has 0 modulation, Or you can try turning off the PL tone and grounding the mic pin.
For keying I would jam PTT to ground, and switch the line that feeds switched B+ to the predriver stage of the PA. You could use just the PTT line but then you would have to wait the 50mS or so for the PLL to stabilize and the radio to make power.
