I haven't actually done any of this myself, but it seems to me that you ought to be able to wire something up where you have a relay that turns the power to the AP on and off signalled by the accessory line to the empeg so that when the ACC line goes off, the AP comes on. But then have another attached to the permanent line so that when the empeg stops drawing power, it powers the AP off. Then just set the power-off time on the empeg to be reasonably long. This way, the AP will usually be off, you turn off your car and it powers up until the empeg powers completely down, at which point it'll go back down. Now, I'm not sure if there's a userland program to power down the empeg when you're done syncing, but it seems like it ought to be possible to write one if there's not.