There is a limit to how high you can set the standby timeout value. That limit keeps changing with each software release, I don't know its current upper limit. Anyone?

You could always wire the player's ignition-sense wire to the constant power line. But like you said, that might be a battery drain issue if the player is left in the car for long periods, as it would never drop out of standby (and you'd get amplifier pops sometimes).

Then there's the theft protection question. Making the player stay in the car "by default" is asking for trouble.