The GCU is capable of interfacing to CAN-bus (a protocol commonly used in modern automobiles) via an on-board MCP2515 chip.
While hardware support exists, software support is not yet provided.
Useful links
Here are some links that may be of help while developing software support (if you are interested in developing support for this, please write a note in the forums):
libcan (German)
libcan (Google German->English)
OBDuino (car MPG/hypermiling display using Arduino)
CARACA (CAn Remote Automation and Control with the Avr), developed can nodes/electronics, currently inactive
SKPang CAN-bus Arduino Shield - Demo software: CAN-Bus ECU Reader demo sketch
the shield uses the same MCP2515 chip as the PCU
I/O Expansion with can-bus:
Can-bus allows many devices on one network and is designed to transmit over lengths of wire. This makes it useful for I/O expansion for even more control outputs than the GCU provides.
These chips from Microchip are available:
MCP25020 - up to 8 digital I/O, up to 2 PWM outputs
MCP25025 - up to 8 digital I/O, up to 2 PWM outputs, and a one-wire line driver option
MCP25050 - up to 8 digital I/O, up to 4 10-bit A/D converters and up to 2 PWM outputs
MCP25055 - up to 8 digital I/O, up to 4 10-bit A/D converters, up to 2 PWM outputs and a one-wire line driver