Id be very interested in this as well. Mremy id be happy to nerd out on this with you. I'd love the ability to set up a secondary microcontroller that can send gcode commands to the duet. I have this idea of using a raspeberry pi or ESP8266 with a secondary touchscreen that I can use to make a custom GUI where I can easily customize a list of my frequently used macros. It cant be that hard. I guess I could do it through the WIFI.
Are there any resources or even better, tutorials, that you have run across that explain in detail how to communicate with the duet from outside sources?
Currently I am in the process of setting up a new PC. Once I finish installing all the tools (IDE, toolchains, etc) I will get back and see what we can do. I do prefer to run a stock firmware as I keep all the firmwares up to date on David's code. Having a dedicated Gcode command for I2C and even maybe SPI would be gold from my point of view.