I went a bit different direction to solve this problem. I already have a 12V power system with battery backup, so I decided to run the Pi from that. I used the SleepyPi https://spellfoundry.com/product/sleepy-pi-2/ to safely shutdown the Pi when the battery voltage drops below the threshold I programmed. When the voltage recovers above another (higher) threshold, it boots up again. I can also read the current voltage over serial for logging, alerting, etc.