Paul, did you ever get the live organ pack when it was available? Yes, they sampled Leslie at fast and slow rotary speeds, but their registration cross faded between the 2. It actually worked surprisingly well.
If you didn't, just try this: put right 1 as a slow rotary, right 2 as a fast rotary (of the same organ if possible; eg legacy AllBarsOutFast/Slow), then edit the voices so that the Joystick modulation + of one is 0 and the other is 127 to create a crossfade (annoying as heck with a joystick, works MUCH better with a mod wheel. Which is why I'd dream of a control on an arranger with both joystick, but also a redundant mod wheel for things like volume swells, etc)
You can hear how it would work. Still loses the gradual speeding up/slowing down on the bare voice, but in a mix, with a mod wheel? Pretty good.
Mark