Just a quick post to share this link that uses an USB Powerbank as a UPS-style system, using a network switch to simulate the power outage (when the power is cut to the Pi and the Network switch the network connection goes down and the Pi detects it).

I could modify this to trigger a safe shutdown when the car is switched off. If there's a way to do it without the network switch that would be bloody marvellous!

http://raspi-ups.appspot.com/en/software.jsp?_utm_source=1-2-2