One option, not pretty, is to use the notes themselves to trigger stop changes. I sometimes configure unused notes for this purpose. For example some sample sets only have 30 note pedal boards and so I use the top F# on my pedal board as a "stepper" to move to the next registration.
Theoretically, depending on the pitch range of the repertoire you play, you could use the top notes of each manual to toggle the stops. You would need to go to Organ Settings, Keyboards and reduce the Midi Keyboard Hi Key for each manual to stop these notes sounding before configuring them to change the stops.
As I say - not pretty, but it may be better than nothing as a temporary solution.
Best wishes
David