The more I think about it, all we really need is a generic System Exclusive message that announces a MIDI transmitter will be using MPE, so that the MIDI receiver can make any adjustments - if any adjustments are actually necessary.

Every aspect of MPE already exists in the MIDI specifications, although some parts are considered optional or are not spelled out verbosely. It seems like it would be much better if MPE did not define anything new at all, but instead just listed the existing features of MIDI that are necessary and then have a SysEx message that allows an announcement (or even query) of full support for the set of messages. Queries would only work with loopback, but the MIDI Sample Dump Standard is an example of a query system over MIDI that requires loopback. This Announcement/Query SysEx could be used by a MIDI transmitter to inform a MIDI receiver that it will be using full MPE, while the Query form would allow a MIDI receiver to assure to a MIDI transmitter that it can properly interpret all required aspects of MPE.

It would be an interesting exercise to go through the current MPE proposal and list the existing MIDI features that already implement each aspect. If nobody else volunteers, I may do this myself. My success demonstrating the Soundplane with full multi-dimensional control of a 1984 Oberheim Matrix-12 seems like proof that the original MIDI specification has everything we need. The only real problem is that the MIDI Spec can sometimes be vague about the full potential of the existing commands.