isn't velocity sensitivity simply two electrical connections ("buttons pushed") that get made at slightly different points in the arc of a keypress, and the time difference between them is measured to use speed as a proxy for how hard/loud the key is pressed? (that was both a question and to explain the mechanism to people not familiar ;)

I wonder if it would be simple ot add a sensor to measure that across the keyboard, and then have it do double duty as aftertouch (aftertouch is an effect that measures if you "wiggle" a key after you've pressed it down) Then his DX9 could be better than the DX7 :)