Unfinished copper fixtures will start to weather and patina as soon as they are removed from the packaging and exposed to the environment.
If you want to have nice, new, clean looking fixtures for your demo's then you might want to use new fixtures each time, or you could try sealing some with a spray lacquer finish. I would suspect that wear and tear on the demo fixtures will damage that finish however.
I would recommend you weather a few copper fixtures for your demo kit. That means leaving them outside in the elements starting now and let them do their thing... Then the client will see just what their fixtures will look like after some time. Besides, you should probably focus on selling the system more then the fixtures anyway. "See the effect, not the source."
Personally, I think the copper fixtures look best after they have weathered for several months and 'settled down' into their own natural patina finish. Others like them best when they are shiny and new.. but keeping them that way is very hard to do.
Best of luck