Thank you for your in-depth reply, but I think I haven't been clear enough.
The one thing we're missing for full MPE support is the concept of MIDI channel 1 as a "master channel" or "global expression channel" as I'd prefer to call it. To get there, we'd add a special module to our framework that simply takes care of distributing modulations from channel 1 to all other channels in the form of offsets to whatever comes in from there. This has nothing to do with Mono MPE, it's just a thing we're gonna do before we slap "MPE compatible" on our product pages.
Back to Mono MPE:
Gathering all information onto one channel sounds like it would work for us without changing our voice management, unless people play two of the same notes on two separate channels. Those would "compete" for the modulations. OTOH I'm not sure if that's an issue since I'd assume a mono patch be played one note at a time anyway, and trills be held a different note to the one playing.
If we don't do it this way, there is no possible option for us to implement a legato between two notes played on different channels - at least not without that major kind of hassle I mentioned.
The channel merge idea has the advantage of simplicity and when the time comes I'd be happy to post a preliminary version of one of our plug-ins for you guys to check out. If it works, good, if it doesn't, we'll probably drop that idea.
I still would need to know if in Mono MPE, the modulations from the global expression channel are expected to be added to the notes played on the other channels regardlessly.