not sure about integrating it into QMK, that's a much bigger project than what we're working on.
QMK already has mouse keys (but that's an emulated mouse), and most of the code for the mouse we've been using is for reading/interpreting the input from the optical sensor, the buttons are secondary/minor programming.
i guess it's something we could look into, but i never thought about combing them as i rely too heavily on a standalone mouse but i see the appeal for an integrated trackball. depending on the controller, space for the programming may be an issue. (some boards don't have enough memory to even support a full install of QMK, ...all the features QMK can do.)
most off-the-shelf keyboards with integrated pointers/trackpads/trackballs have a keyboard and a mouse cable. even though they are in the same housing, they are actually wired and interpreted separately.
currently, we've been using the old teensy (but promicro should be fine too) with the optical sensor iso mentioned to make a custom mouse.