Quite possible, I guess, since the Reaper MIDI editor already is an extension plugin. It seems that maybe not all the required APIs are yet public to implement such a thing. But even if/when those were all available, it would be a big big task and require lot's of experience about MIDI and especially GUI-programming. Not something that could be whipped together in one night...Or even two...
Maybe if Cockos open-sourced the already existing MIDI-editor, the needed work would not be so much.