I've got an idea for a simple pedal that converts (electronically) a regular expression pedal movement from a 'sine wave' sort of waggle to-and-fro, to a 'sawtooth wave' movement. As you rocked the pedal forwards, nothing would happen until near the end of the travel when it'd suddenly 'drop off a cliff', followed by an automatic rise up (with speed of 'release' adjustable).
It'd have a 'depth' control to decide how much of a drop there'd be, and a manual pushbutton switch to bypass it (back to regular expression pedal operation).
It wouldn't work on its own, but you could use it with any kind of pedal that had an expression pedal input. Just put it between your expression pedal and the Moogerfooger lo-pass filter, or the Strymon Mobius, etc etc...
If it sold for about £70???
Or you could switch it to work with audio, and using only your expression pedal (and guitar plugged directly into this new box) do the volume-ducking sound like the Minor Concussion does...
That might have to cost a bit more...
Whaddyathink?!