What's involved with getting Komplete Kontrol to update it's focus to the currently selected track in Reaper? Does this require Native Instruments to add support for Reaper? Or Reaper to support the Kontrol keyboard? Could this be done via scripting within Reaper?

Or is this where OSC comes into play?

OSC tool is only available for MAC. But for example the Python scripts from Ableton for Komplete Kontrol could be used for WIN/MAC.