Ive already commented on this here:
there is already low level support in the STM firmware, and this has been extended up to the axoloti firmware.
However, we have not exposed it i.e. to get to the hid_buttons level which objects can use.
it would be pretty trivial to add, and certainly would work for your needs (i.e. just outputting ascii or keyboard codes), but for boarder needs it gets a bit more complicated.
if your interested the file is:
and the structure is defined in:
at the trivial level, is just a matter of exposing the structure, and then creating an axo object to use it.
one small point, the firmware has a limitation of: AZERTY or QWERTY mappings (and the firmware has to be compiled differently for each), and a max of 6 simultaneous keys.