hello,
the adjustment knob on the side of my expression pedal is always at the zero position, because i want the re-20 to produce no echo if i do not step on the pedal. if i want it to produce some or maximum echo i just move the pedal to the desired value and leave it there. so i may be removing the value poti as i do not really use it.
the cable of the ev 5 is the real weak link - mine has been a constant hassle as it seems to be broken somewhere and was not functioning all the time, what is a pity in a busy live situation, because it causes the echo to stay on.
so i drilled a hole into the side of the housing and put in a quarter inch stereo jack and can use a detachable cable now... much better, and because the cable is now on the side i even can rest the pedal aganst a wall to prevent it from slipping away.
i imagine the space in the re-20 would be enough to fit a poti in the same place as the expression pedal jack is now, but it would be a bad place because it is at the backside of the unit between the in and out cables.
so it would be better to solder a pot to a short cable with a trs plug on the other end to position it a little better, like you said.
also it might be good to think about really using a foot pedal, because then your hands are free to control two other knobs which is really good in live performances.
the pots in the ev 5 are a 10k for the pedal and a 50k for the min/max value knob.
here is a pic of the schematics:

peace, jo