I have tried to make the "joystick" contol as described in "4 Way Crossfader with Max in Center" to control the volume of each of four zones on a Casio PX-5S. This is somewhat similar to what I thought was requested in the original question.
No matter how I try, I can't get it to work that way. What I get is: all four zones at full volume in the centre of the grid; two zones at maximum (127)and the opposite two at minimum (0) at each corner of the grid; and one zone at '0' and the other three at '127' at the centre of each side of the grid. Nowhere can I get one zone to be at maximum and the other three at minimum.
I also tried with the "XY Pad to Control 4 Mics" without any success.
Logically, I can't see how this could work as requested, but I am hoping I am wrong and there might be a solution.