On my diesel I want to add an exhaust gas temperature. This data is not available from the OBD2 interface, you have to install a standalone sensor and gauge. What Im trying to figure out is how I could get that data into autopi for the purpose of showing it on my in vehicle screen.
So you need to figure out how to interface with your sensors. The AutoPi has a number of ways to interface external sensors. Besides the USB ports, there are the GPIO pins, see here for specs on each pin: