Robert,
I'm interested in producing Regs. that for example would change from Split Keyboard to Full Keyboard playing without changing Style or voices, not too dissimilar from your requirement.
The Style can be tackled as you suggest by saving a Reg. with "No Style" selected. This special style has to be a copy of any style & saved to User or HDisc or stick memory so that it can be deleted once the Reg has been saved. Then when the Reg. is activated and it can't find "No Style" it leaves the currently playing Style in place. In theory you could do the same with Voices, but I need help in thinking this through- because although we can copy a voice & call it "No voice" to a deletable User or Disc memory the copies in this case only act as shortcuts or links for Tyros back to the appropriate Presets. So the copy can be deleted after storing the Reg. but Tyros still finds the Preset voice the copy is based on & changes it in spite of! We would need to delete the Preset (not possible) and even then I'm not sure that Tyros wouldn't replace the wanted voice with the missing-voice icon & silence! So unless someone comes up with a way round this we're left with having to use the Freeze button (set up to freeze voices, which may not be convenient)if we want to achieve no change.

John