matrixctrl and parameter_mappable

Hello,
I'm trying to make every button / knob at the intersections of the rows and columns of a matrixctrl controllable by a dedicated MIDI cc: message from an external hardware controller box.
I.e. in the case of a 3x3 matrixctrl I'd like to use MIDI cc: 1-9 to control every individual button / knob.

Yes, I did read about the parameter_mappable attribute which (to my understanding) should allow exactly this. However, somehow it doesn't work (=I'm doing something wrong), meaning when I click on the Assign MIDI Map icon at the bottom of the patcher window, the matrixctrl object is not purple.

Could somebody please give me a hint or just post here a matrixctrl with a properly applied parameter_mappable attribute (if this is the proper way to do it)?

Thanks a lot :)