You could do that by having a 2nd organ part of the same sound with that key range but muted and then store a scene with it unmuted. I've done this a few times to change key ranges in the same performance. It works well when you are adding something as unmuting a sound isn't audibly obvious until the new sound is played. Going the other way isn't as successful as mute will cut the sound dead instantly (as you would expect). In that instance I usually use assign switches to turn elements on and off.

Hi Phil, Thanks for your reply. Your suggestion works well, but only when empty part-slots are available. Playing live, the 8 slots are a bit of a limitation. My montage 8 is usually split up in 3 or 4 ranges and my keyboard is expected to offer piano, organ, synth-bass, strings and brass, and sometimes arpeggiated percussion or other effects like a vocoder. Then I often layer multiple instances of these instruments. E.g. a typical Rhodes needs 2 parts, The Bosendorfer uses 4. Brass takes up 3. It would be welcome if the 9-16 parts could be used simultaneously.
With only two hands (and fortunately a Boss-RC202-loopstation) I imagine the best way is to buy a second synchronized montage?
Seriously, I would love contemplation on my request of storing different key-range-limits per scene in a new software update. That seems programmable to me.

I appreciate what you're saying about running out of free parts - I agree that only 8 under keyboard control can be limiting. There is another option rather than using part note limited parts but it depends how many elements your organ part uses. If it is 4 or less you could programme the key ranges into 2 different sets of elements and use XA control and the assign switches to turn on the elements that use the lower register.