I would do pin 12 on the KCT-72 for ignition sense.
I've tried to get Kenwood to support an "external speaker" via the OSP (PA speaker) ports on the KAP-2 interface with no dice. The "external speaker" is using the ESP connection on the KAP-2 and can be switched on and off. However, this is useless if you have Multiple Audio Output selected and intend to actually use the KES-5 speakers as intended in that mode. If you don't care about hearing all decks at once via Multiple Audio Output, you could connect an outside speaker to the external speaker outputs of each deck (three speakers) and program a button for "External Speaker" and toggle the speaker on and off. However, in Single Audio Output mode, the output of the KCT-72 speaker connection is switched not mixed audio, with priority given to Deck 1 (not the selected deck). I can't recommend it.
Yes, these radios lost functionality compared to a TK-x90 / 5x10 platform.