I am working on a standalone solution, that pretty much does this, using MIDI still.
Alot of MIDI Drum Set triggers are designed in a way that you can't just wire them directly to the RockBand Set pads. As the pads are pezio(sp?) triggers, they actually create voltage to trigger, where as (my) drum pads for my electric kit close a circut instead of creating voltage.
Not sure if that makes sense. Anyways, the only one that I have seen where you can just slap a pad on it and it works, is the bass kick. That one is a actual switch, and not a pezio trigger.