Sorry, my previous text said "I don't think you can select registrations using a midi message" but now I read your post again that isn't the question, is it....

I think Yamaha use CC32 as "Bank Select", but you need to follow that with a standard midi program change and I don't know whether your accordian can do that.

To select voice in the Left, R1, R2, R3 parts could you assign them to different midi channels? Trouble is you'd have to get the accordian to send out bank, program and note data on multiple simultaneous midi channels.

I fear that what you want may not be possible to the degree you would like. I've investigated driving my PSR3000 from external master keyboards and concluded that whilst it was easy to send notes and program changes to one or two midi channels after that not much was possible.