OK, that was always the problem if a plug-in likes to do this for itself.
This was already the case with VST2.x and one of the reasons why we think that this is a host responsibility.
Just think about a merged MIDI input where two or more actual hardware devices will control a plug-in. A host can make the distinction from which hardware device a MIDI CC change was sent and can map a parameter to it. A plug-in does not have a clue which device was used.
The same for other controller devices which may not work with MIDI at all, for example a Joystick. A host can map such things a plug-in not.
So, maybe a possibility would be for a plug-in to get a display string from the host with the information how a parameter is mapped, but not a MIDI-CC number.
Cheers,
Arne