Yes, you can program the “Rotary Speaker” Effects “Speed Control” parameter with any available physical controller.

Some will work better than others but you can certainly assign it to a different controller than the MW.
The MW was selected because its behavior is most like the original modification — back in the day when the Hammond organ company was sueing the Leslie speaker company, the half moon “Chorale-Tremolo” switch attached to the left side of the keyboard. Very much like a MW except the movement was left (Chorale) slow and to the right (Tremolo) fast.

From VOICE Mode
Press [EDIT]
Press [F4] CTRL SET
There are six Control Sets, two to each tab [SF1], [SF2], [SF3]
You will find a Control Set that sets the “Source” = MW, and the “Destination” = InsX SpdCtl
The Insertion Effect (A or B) Speed Control
This is how the MW is set to control the transition from slow (MW at 0-63) to fast (MW at 64-127)
You can change the “Source” to different physical controllers.

If you set it to FS Foot Switch, you can use a Foot Switch plugged into the Assignable Foot Switch jack on the back panel
If you use a momentary foot switch it will behave like a sustain pedal. That is, 0 when disengaged and 127 when engaged. But momentary means you must maintain contact with the pedal to maintain 127.
If you use a clickable foot switch it will behave as a latching pedal, each click toggles the parameter setting.

Fantastic! I'll try that in a few minutes. Appreciate your help.