Hi Bernie,
If you already have a MIDI template setup for use with the SD40, I assume that you have saved it as a User template on the USER drive.
Once you select a MIDI template, it will remain loaded even through power off/on cycles, and if you want to select a different one, the MIDI template screen will always open at the page containing the one currently loaded.
So why not save another user template containing the MIDI OFF setup that you want to use. This will be saved to the same page as the User template for the SD40. There is a preset called MIDI OFF that you can start off with, and either edit, or use without modification if it already does what you want, but whatever you do, save it as a User template (to avoid confusion with the original preset, give it a new name).
Then whichever one you are currently using, when you press Function > Tab to Menu 1 > MIDI, the selection screen will open at the page containing both of your user templates. It will be very easy to select the one that you want since there will only be 2 templates listed!
Although this only saves you from having to move between the Preset and User pages in the selection screen as you would if you use the preset MIDI OFF template, perhaps you would still find this less cumbersome.
It would be nice if there was a Registration Group for the Midi template, but as far as I know, there hasn't been one in any Yamaha arrangers to date, including Genos, where the range of parameters that can be included in a registration were significantly increased, so I wouldn't hold your breath!
However I would have thought that using a MIDI switcher box or some other hardware would not really be any easier than just using the keyboard.
Regards
Ian