Thank you for the thoughtful responses. We ideally wouldn't need the Arduino if we were able to control all of the expansion pins with C like the Arduino. Is this possible? That would help us decide which path to choose.

We are looking to control any unused or expansion pins as I/O pins in order to tell the Arduino when to run and receive feedback back to the duet.
What's the best approach, should we try to remap the pins in the firmware, or simply use the current mapping and gcodes for different purposes?

adruino sbc headers output