It looks like there is a product that does the conversion from Lightning to RS-232 already.
Lightning Serial Cable for iOS
If it supports the sync bytes that are needed to talk to the Z, it will translate the USB communication from the Lightning connector to the UART communication the car needs. I emailed them asking if it supports this. I will post their response. Using this adapter, or building one like it, it should be very straight forward to get iPhone 5 devices talking to the car like other older iDevices with the 30 pin connector.
