I haven’t been able to find a spec on the expression pedal interface. However, I have now determined that it is incompatible with the Korg EXP-2. When connected via a TRS cable the Korg measures a constant value of 100kohms between tip and sleeve, with the wiper of the potentiometer connected to the ring of the connector. ie; ring-sleeve resistance varies from nominal zero to 100k as the pedal is moved from min to max (toe down) position.
I have deduced that the maestro is expecting the pot wiper on the tip of the connector. Calibration doesn’t allow for different connectivity, just different values of resistance, so you really should publish a compatibility chart for this to be most useful.