I have the Montage as my lower tier keyboard, Kronos on top tier, Nord G2 Engine for good measure, and a small NUC computer in my Gig Rack running Top Ten Software's Cantabile Performer, which is my MIDI Processor and Live VST host. I need to be able to play Montage Parts from the Kronos and Kronos Timbres from the Montage and vice versa, and both keyboards need to drive the VST instruments I have in a song and the Nord G2 Engine.

My MIDI setup therefore is a "star configuration", with the computer as a hub, so that all MIDI goes through Cantabile. Therefore, I get around the Montage MIDI limitations by leaving it in single channel mode and any MIDI channel mapping/splits/layers to/from the Montage are handled by Cantabile. If you have a similar need to mine, the free version of Cantabile lite is well worth checking out, or you can try the paid for versions for 30 days for free before you need to license it.

This works for me, however, I think Yamaha should fix this limitation for musicians who do not want computers on stage but need to integrate the Montage into a keyboard rig where it is not necessarily the Master Zone Master mode driving external devices, but Performance Parts that need to be driver from external devices. If my Yamaha EX5 and Motif Rack ES allow you to allocate any receive channel to any part in Performance Mode (EX5) or Multi Mode (Motif), and of course my Kronos, then why not the Montage?

this issue (and some another) is insane, this issue is deseperate, and the fact to not have a clear answer from yamaha team designer is for me the sign that i may leave the boat before it sink.