The idea is that I'm working on a Nexus 7 install. The USB rom on there has provisions for a sort of hardwired setup, whereby the Nexus will be in host mode but still draw power. One config option is, if you remove power, the Nexus will go into deep sleep based on the assumption that this corresponds to you turning the car off. This way, the Nexus doesn't drain the car's battery, and in deep sleep it'll last a solid 5-10 days before its own charge is out.
The problem is terminal 30G turns off a full half hour after the car shuts down. There are many reasons that coding this differently, even if possible, would be non-ideal.
What ends up happening is, you unlock the car, 30G comes up, nexus turns on, got your nav/music/whatever, all is happy. Get to your destination, turn off the car and walk away: the nexus is still on, glowing like a neon "steal me" sign, and if nothing else it's draining both batteries unnecessarily. Ideally, it'd enter deep sleep as soon as the radio shuts off.
Listening to when my sub amp pops in and out of power, though, I think that the remote line from the headunit doesn't EXACTLY follow its own power state, for whatever reason.
If that's the case, I'm going to have to do something very silly like open the headunit and MAKE a remote signal sourced from maybe the panel LEDs or something. That'd be really non-ideal.