Mojo, here again, as manifold vacuum is max (and airflow is minimum) with the throttle closed, any time you remove your foot from the accelerator, your compressor would disengage (with whatever system you use to do that).
Just a thought, but there are times when, at a dead stop, I shift into neutral to reduce the transmission (actually, torque converter) idle load (to save some gas. Maybe not much, but every bit helps [especially for the '74 vette]). The neutral position switch could be tapped for that condition.
Might be a pain to implement a connection, but that action would for sure be an indicator of a dead stop (555 timer OFF, relay OFF, compressor OFF) and, then, after a return to drive (timer ON with a 555 driven 5 sec relay ON delay, then compressor ON again). This would also work from a "cold iron" (car off) situation.
I'm sure there are others ways to skin this cat (my insincere apologies to PETA).