@nick: You could accomplish the sort of thing you're talking about by using a trick that was employed in old organs.  Use a photoresistor in place of the pot and wire up a light.  This light doesn't necessarily need to be built into the same circuit as the echo bender, if you'd like to avoid that sort of complication.  On your pedal mount a board with a "V" cut out of it that will alter how much of the light makes it through to the photoresistor.  Photoresistor on one side, light on the other, instant expression pedal!