Got a close look at the schematic, and indeed the external "mic amp
output" connector pin 5 of accessory jack is basically the same as the
audio output of the microphone itself, useful for DTMF insertion but
not useful for CTCSS. It appears likely I can attach near/to the output
of the deviation control R115 which feeds through a series resistor and
capacitor to the FM modulation varactor D32. This looks like it's near
the center of the lower PC board in an area that's not too cluttered.
I'm not sure if I'll need to inhibit the tone on receive or not; the
circuit appears dedicated to transmit mode only.

Again, thank you very much for the insight - it would not have been
possible to start this project without your kind help!

73 Dave WB0GAZ