Isn't this already supported in most modern MIDI keyboards in the form of Aftertouch? If that's not what you're looking for, you could always get one of these:
http://www.cerlsoundgroup.org/Continuum/
That's exactly what I was thinking of. Thanks.
(Of course, at >$3000 I'm not actually going to buy one. Just geek out over it.)
Not MIDI, but CV: Ondes Martenot / The French Connection