Eeee Thank you! Wow, that makes me happy.
The filter cutoff is being automated in a slightly hacky way— It is possible to give velocity influence over several different parameters for each partial within a timbre. So my approach has just been to assign it to the filter and a slight bit to the pulse width usually, while giving it no influence over loudness. Then using expression to automate the loudness instead.
If you are interested to explore some editing, SoundDiver has a really nice panel for the mt-32, it can be found pretty easily.