One complication I thought of, is "what MIDI channel would I sent the message on"? The RotarySP button automatically "knows" (since it's part of the system) which channel (L, R1, R2, and/or R3) to work with, i.e. which of these have OrganFlutes sound selected, with a Rotary DSP effect in place. So, pressing the internal buttons (or assigned footswitch) must internally "do the right thing" for one or several MIDI channels, as needed, to control wherever there is an OrganFlutes sound.
Not sure how this would be handled externally, unless it's on the MIDI channel for "Keyboard", which would then be internally "translated" to act just like a press of the RotarySP button.
If only we knew the internal implementation details!
I have contacted Yamaha in several ways, so we'll see if someday this ends up added as a new feature!
Thanks to all for the comments and ideas you've put into my this thread that I started ... appreciate all the thoughts and comments!
Jim