Pteron has now been merged into QMK and is available here if anyone wants to have a look.
On a related note: It turns out that 5 rows by 12 columns is about the largest keyboard that’s possible to build using a Pro Micro, without getting into some shenanigans with the matrix. I’ve used all available pins, the only free are 3 ground pins, RAW, RESET and VCC.
#define MATRIX_ROW_PINS { D7, E6, B4, B5, B6 }
#define MATRIX_COL_PINS { F4, F6, F5, F7, B1, B3, C6, D4, D0, D1, D2, D3 }
So I got lucky when picking the number if keys I wanted on the board!