i have a similar problem with my dell streak 7 which has even worse standby time and will drain overnight even in airplane mode with the screen off.
here's my idea, pick up a smart DC-DC powersupply with a conffigurable low voltage shutdown and wire it to a constant 12v supply, trigger your standby using your headphone connection to your audio source. that way you keep your unit charged without ignition and still protect your battery. not sure how long it would extend the standby life but it couldn't hurt.
only other idea would be to find a power supply with a auto boot on ignition signal and hard wire it to the tablet.