I'm that US-428 guy. I posted a pretty long one on this here:
http://www.sagantech.com/metroforum/viewtopic.php?t=34
To summarize, I'd guess the quickest way to allow even more support for controllers is to allow the controller to be associated with a particular interface. It would then allow good fader mapping without taking up a channel on all interfaces. Supporting the JLCooper CS-10 protocol would be my dream solution.
Scoot - my guess is that it is more likely for Metro to support controllers.
Just a crazy idea that I should probably spend more time thinking about before typing it but... If Metro allows AppleEvents to drive its controls it would be relatively easy for a third party to write an app that could accept the MIDI messaging from the control surface and forward it as AppleEvents to Metro. There are complications in Metro responding if the control was successful... yep, I should have thought more