@The_Elf It should not be so difficult. From the schematics I have, it seems that the RING voltage goes (through a 1kOhm resistor) directly to the input of a ADC0809 (an A/D converter with an input resistence of 2kOhms). So a simple linear potentiometer, used as a voltage divider between ground and the 4V tip voltage should do the trick (the variable voltage pin should be connected to the ring).
As I said before, I have also a Roland D50 which I use as a master. I have two Roland EV-5 pedals connected to the D50, so I took one and tried it on the MK88…if I set the “Minimum Volume” knob to zero I am even able to see some changing values in the midi monitor (otherwise it always produces the maximum value). Unfortunately, even with the knob at zero, it reaches the maximum value quite soon…so you can’t exploit the full range of the pedal.
I had a look at the EV-5 schematics: it seems that the tip is connected to the variable voltage contact of the potentiometer and the ring to the highest voltage contact…for the ELKA MK88 it should be the other way around. So maybe inverting the ring and the tip in a EV-5 pedal could do the trick. If you can get one and you are good at soldering you could give it a try.
Anyway, maybe we should move the discussion to private messages…
UPDATE: I was curious, so I did it myself. Inverting the ring with the tip in the EV-5…it seems to work! (with the minimum volume knob on zero).