@arachnaut: Wow, you keep coming up with these CW applets that I haven't seen in so long. I'd forgotten they were there! Does this work in real time? I looked around and could only find very basic record filtering in Preferences. Nothing else that looked like real time filtering. When I needed something like that, I had to use external devices, like the filtering in my MOTU Midi Express XT midi interface or boxes like the MIDI Solutions Event Processor.
@windplayer: Midi Solutions also makes something called the MIDI Solutions Breath Controller Interface Box which might be perfect for you.
Checking around, there seem to be Google entries for Midi filtering software, but what I saw looked like homebrew solutions and I couldn't tell if they were real time or not.