I've only used a standard TRS cable with my Quad and CC pedals, they work just fine. I never knew about proprietary ADA cables.
In fact, I've been using standard TRS with all the ADA switch pedals.
You know, there could be something in the MXC controller that takes the signal from the Quad and CC pedal, and translates it into a code to send to the MP-2. Since it's the only one that does all those functions in the MP-2. Then again, I remember experimenting with the Ground Control pedal when I first got it and I did plug in the EB Volume pedals into it and controlled different parameters in the MP-2's programs. It was a much more involved set up procedure if I recall correctly. So much so, that I never bothered again and just used the MXC after that.
I did open the CC pedal to get to the pot to spray some Deoxit into it, and that's all there was in it. A pot wired to the jack. (A membrane pressure switch in the toe of the pedal).