The reason why Filter Amt is not a CC source/destination is that it's actually a macro for the mod matrix Env-to-Cutoff point (it would technically be possible to give it a CC, but at least for now it works as intended with no CC). The matrix mod value doesn't really make sense as a CC since it's not a single parameter (can be for any matrix point) and not always active.
There is a workaround for controlling the Filter Amt from the keyboard or a sequencer: Assign the Filter Amt knob to one of the user assignable matrix columns, the assign Press to that column and crank up the mod amount. Now aftertouch can be used to modulate the Filter Amt. Not quite the same as having a CC for it, but it may be the next best thing.
