So I was looking through the REAPER file folders, and found a few MIDI Joystick control methods, "Generic", "Rockband_drums", "Rockband_Guitar", "Rockband_Guitar_Easy_Ps3" and "Rockband_Guitar_Play_360". All of them are .txt files, and can be loaded in REAPER's MIDI Input controls. But all I've found out how to do with these, are to make some blippy noises, and that's all. The text files document how to select difficulties by holding down select and pressing buttons, etc.
TL;DR: What's the purpose of these? Just to have some fun little extras to play with? Or are you planning/already able to do on-the-fly note-authoring with the instruments...?
(Also, wouldn't it be totally awesome if somehow, someway, we could play the parts in the preview window, without using audition mode? xD I originally got excited when I saw these text files for that exact reason, but then quickly looked at it realistically; no way you could fit the game engine into a filesize THAT small. Plus then they'd be distributing RB for free, with REAPER.)