Don't have my empeg here, but I'm guessing you've already looked at /proc/empeg_*? I think the files mightn't appear unless the device is actually there so you might need to look while docked in the car.

This is all pure speculation of course, but I know you can read things like headlight sense etc (/proc/empeg_power?). Wait! ... depending on how dodgy your AC sensor is you could boot, check the AC switch status and then see if the .ini option worked. Pretty clunky - surely there's something easier.