@mhorgan, did you find a way yet to get precedence over AppleMIDIUSBDriver.plugin and possibly com.apple.driver.AppleUSBAudio?
@Apple: wouldn't that be easy to answer: just a yes or a no. And if you don't care, just a "we don't care" so we know at least what to expect!
I haven't gotten that far yet. Trying to wrap my head around what the user mode code would look like. Is it essentially and an Audio Server Plugin renamed or something else? A sample or some documentation would be helpful.