Brennan stated this about a year ago: Use of the expression pedal outlet?
In responce from this post earlier in the same thread:
" csapoa
Hi Brennan, the expression pedal is a passive circuit, which means it has states: variable resistance (/diodes) in between the T,R and S. So functions could be assigned to states of a volume expression pedal or just have a custom built switch for any of these states. Then it is ‘just’ programming. This improvement would mean that you don’t have to change the circuit only the software to have a foot switch programmed to a parameter. UNDO!! 
"
I think that now SS is working on full Midi, It’s also the time to at least add vol EXP and 2 button switch option to the EXP Jack. Now is the time to make it a priority, in tandem with Midi.