Currently I envision a hard top to be a two piece affair. One piece over the cockpit with a vertical rear window, and a second piece over the rear deck lid purely for cosmetics. Either that or no trunk access with a one piece top.
I don't know what the rear deck lid is going to made out of, or how it will be attached, but it could be replaced with an aftermarket hard top deck lid that removes the seat fairings, accomodates a hard top and provides a trunk lid that hinges in conventional manner. If the factory hinges for the deck lid are designed right, and the deck lid is of a plastic composition, changing deck lids might not be too bad.
The fact that the show cars had no top means to me that Pontiac is mulling these things over right now. I think they have something very distinct in mind. I don't put much stock in the top designs on the test mules. I think they were rigged up just to provide shelter in case it rained while they were in England. They might be a first attempt, but I doubt very much that they represent what the finished product will be. I am hoping that Pontiac realises how important the top design is, considering how much effort they have put in on the sheet metal.
Bob, if your reading, get down to the studio and make sure they don't mess your baby up!