I agree with DaniH.
I think Rolands solution of 'swapping' is quite elegant and the change is instantly.
Since I do a lot of switching between the synth engines, especially in Performance mode, I don't see any benefit from a overlay.
However, I do think that a computer/iPad editor could be very handy to have a overview of your patch.
When you switch the plugout, the editor will to the designated page in the style of the corresponding synth.
Well, that's what I like to see.