I'm lost. Not quite sure what your saying. I have never used the accessory AC jack on a mac, so i dunno.

But you say when the unit comes on, this secondary AC output is switched? kinda like an old stereo?

If so, then a simple 12v relay would suffice, drive the coil from the 12v of the power supply. power supply comes on, relay clicks, and sends the 120v/240v from the main line in to the accessory line out.