@J... my understanding of OP is the same as yours i.e. having to hand a traditional Slow/Fast switch.

The Mod Wheel, due to its placement and ergonomics, would be suitable, if not ideal. You just use it like a Gear Shift... full down or full up.

I notice on other "Organ-centric" KBs like the Roland VR and the YC61, the Fast/Slow is a Toggle Button.... like the MODX Assign Switch.

I would like the OP to confirm that he cannot use MODX Part 8, because all Parts 1-8 are already in use, in most or all of his prepared Performances.

Skim reading through this thread again, the only reason I could see that OP can't use Part 8 is because it would be a ball-ache to have to go back and re-edit his existing Performances.

Historically, this seems to be the result of being advised to use Part 9 at some point in the Past, which he has followed religiously since... and is now experiencing the limitations of Parts 9-16.

(FWIW... I only ever regard my MODX as 8 Parts. I presume 9-16 to be primarily unavailable, "last resort" Parts, that could be played by a Sequencer/DAW if I had expended Parts 1-8. I have never managed to need/use more than Parts 1-8 yet).

If I was OP, I would be tempted to bite-the-bullet, go back, and move all Part 9s to Part 8.

You click on Part 9, and instead of clicking EDIT, you click on "COPY". When the "COPY" screen appears, change it to "EXCHANGE" and change destination to Part 8. Then STORE the Performance.

It literally takes seconds, per Performance.

Then go back to the NORD and change Global MIDI from Ch9 to Ch8.

There maybe some additional MODX MIDI setup caveats to take care of, I am sure these can be quickly addressed by the residents here.

The fundamental benefit here is that the user has more options and more control on Parts 1-8, than on Parts 9-16.

In Part 8... the Modwheel is available. So are Assign Switches and so are Footpedals.

So in addition, you can now split functions across more Controllers, and not try to cram them all into a "limited set".

I'll take this opportunity to apologise if any of my assumptions or advice is wrong. My approach to the MODX is always to look for the most simple solution within the MODX's normal and intended functionality. If I encounter an issue where only complex, elaborate, semi-functional solutions exist, I usually go back and rethink/prioritise my needs and design... as in "do I really need this?", "Can I achieve similar results a different way?".

To wit....

Does it absolutely need to be in Part 9... excepting 30 minutes of re-edits?

If Parts 1-8 are full, is every single one of those Parts vital? More so than the Organ?

Can any of Parts 1 to 8 be condensed?
Example 1 Part has 1 Element, another Part has only 4 Elements. You can have 8 Elements per Part, which can be split/layered per Element. So you can combine those 2 Parts into 1 Part and free up a Part Slot*.

* I did this in one scenario... instead of 3 Parts of incidental instruments, I combined them to 1 Part, each instrument was still Playable on its own Key Range. I had to rethink the Insert Effects, and find a "one-size-fits-all" solution... which I did, and it was an acceptable compromise. By contrast, not being able to fit in a Hammond B3... was not an acceptable compromise.

😉

Refactoring Parts 1-8 into less Parts is a possible option. The impact on insertion, system effects, and other Part globals will be something to deal with as mostly already summarized previously.

Also as an option is to do the "wasteful" and "complex" envelope follower idea - but by not consuming any Part in 1-8. This would mean combining two complex solutions.

1) Pick any Part in 1-8 that would have keyboard control ON when the Performance is initially recalled and also when you want to switch the rotary speaker speed. Also, this Part should not have a constraint that the MIDI output channel cannot be changed (i.e. is not currently controlling some external device AND if controlling an external device that the external device cannot change its MIDI channel to accommodate)
2) For that Part, setup Zone control so the MIDI output channel is (say) channel 16. It can be any unused Part (channel) in Parts 10-16.
3) Use one of the two "loop back" options. Either using standard 5-pin MIDI because you're not using the USB TO HOST connection or use the software echo-back (through) so that MIDI channel 16 (or selected MIDI channel in step 2) messages are sent back to MODX Port-1.
4) Add the FM-X "burner" Part mentioned before to MIDI Channel 16 (or the selected free spot in Parts 10-16). Follow the previous tips about setting this up. Note that the input controller on Part 10 (or your selected burner channel) for Volume could be the Assignable Switch 1 (or 2). Even though this is Part 10-16, since we are using a "loop back" option - this channel will get all messages from the Part 1-8 that is sending MIDI to this channel including piano keys, assignable switches, etc.
5) Add the appropriate envelope follower replacement for the original modwheel controller to Part 9. This would use Envelope 10 if your burner Part was chosen as Part 10.

So by piling up complex over complex -- you could have an assignable switch eventually signal a Part that has keyboard control OFF (including Parts 9-16) if you elect to burn a Part in 9-16 to generate an output for the envelope follower to use and also ensure some Part under keyboard control (must be ON) in Parts 1-8 which will have its (Zone enabled) MIDI output set to the burner Part's Part number.

The other constraint here is that you can't be relying on single/hybrid since using Zone will automatically place the Performance into Multi-channel mode defeating any other mode.

I didn't say you should use this -- that's for you to choose. However, this combined idea takes away one demon (burning a channel in 1-8 ... moves it to 9-16) at the expense of some additional complexity.

I have tested this. I used MIDI (5-pin DIN) loopback. I had Part 1 as my Zone part with its MIDI CH output as 10. Part 9 was my organ simulation Part. Part 10 was my burner FM-X Part that used [ASSIGN SW 1] in order to change its volume output. I was able to see that the A.SW1 was able to modulate Part 9 (I used pitch -- but you would use your own destination parameters). In other words, it works.

I do NOT have 1-8 parts used, so Antony's point is a good one and maybe I should go back and change all the other performances from part 9 to part 8 for MIDI communication with the Nord. Indeed the original point of using part 9 for MIDI was to reserve parts 1-8 for all of the additional available functions, but it's true that I have never used all 8 parts yet in a performance and, quite frankly, I'm not sure I ever will. I play more traditional, "vintage," instruments because I generally play 60s and 70s covers. That said, I definitely consider myself a novice with the MODX and still spinning my head trying to do basic things, so it is possible I'd "grow" into needing all 8 parts later on.