Thanks for the tips. So something like:
12V battery -> 12V diode -> capacitor -> adjustable power supply at 8V -> Arduino vin
I'm not that concerned about resetting from a user perspective, just from a prevent-damage-to-the-arduino perspective. None of the information is mission-critical (sounds similar to your usage, Doug), my plan is to turn the chip on only after the engine has been started. But of course mistakes happen, so better to be prepared so I don't fry it.
Thanks for the suggestions!