This is very interesting. Not the first thing that I would have thought of for rio-arduino communication. The roboRIO has a second I2C on the MXP, but I can understand not breaking that out. However, I2C can have multiple devices connected as a bus, allowing one to connect multiple slave devices (Arduino, Pixy) to a single master (or even multiple!). Just a note
This is super unique and it's obvious that some good problem solving was had. How did you do it on the roboRIO end?