[rzzzt]

A mapping for the keyboard matrix can be "discovered" by pressing all keys in sequence. There are lots of AVR-based projects that emulate an USB HID for a custom keyboard matrix or other input; for an example, see [1][2].

[1] https://deskthority.net/wiki/Easy_AVR_USB_Keyboard_Firmware

[2] https://geekhack.org/index.php?topic=51252.0