I have a Ford Escape Hybrid, and I too want to put a pc in my car, but I want to maintain the hybrid functionality.
Since your vehicle has the Nav system, it has Ford's ACP module (Audio control protocol). This is an RS-485 protocol. I have begun decoding the messages on this interface. Is there any specific reason you would prefer ODB-2?
Using my approach I will be able to connect three additional wires to the pc, via a serial port, and gain the instantaneous mpg, the battery state, and the mode the vehicle is driving in. Software would then be able to determine the average mpg. I could start looking into the odb-2 method, if there is a good reason. It just seems like a lot more data, thus more work.