For Mk2, OSC is the way. And yes, the only semi-public description is in the plug-in you mention. You do not need to know java... just understand to which port you need to connect OSC and which initialization message to sent. The rest is as for MIDI.
But there is MIDI implementation, common for all keyboards (and it is not Mackie based). There is documentation, but again not public. There is open source plug-in for REAPER which use it.
I have too many projects on hold and fancy project which I try to write at the moment ("auto-accompaniment", as known from many keyboards, but I want that as a DAW plug-in. Fixed build-in styles in my Kawai DP are too boring
). So I am not ready to dive into that area myself (and I do not have NI keyboards).