This update has been delayed a little due to other commitments, but I've finally finished converting the keyboard scanning software into Arduino libraries. There are currently two versions: the base version is for use when connecting the keyboard via GPIO pins (download link) and there is a second version for connecting via an MCP23017 I/O expander chip (download link).
Both versions come with an example program which will appear in the Arduino IDE's examples menu when the library is installed. The base version has no dependencies but the 23017 version depends on both the base version and the Adafruit 23017 library, which you can install from the library manager.
My next task is to update the main download (the one with the software, labels and 3D models). Hopefully, that won't be delayed as much as this update!