Hey Dave,
Version 7.5 will have the ability to have its own instruments setting defined to Plates Mode. So if you set an instrument group to show none, when you enter a Plate or Airport Diagram, that Instrument group with no instruments will become active. Yet when you go back to a sectional, that instrument with instruments will be re-actived.
Basically, each of the (Landscape, Portrait and Plates & Diagrams modes) will have their own instrument setting that reside when that mode is entered. I think this will do what you are looking for.