What did i see? Black mustang convertable.

Ford is making a push into the aftermarket parts industry. In fact, they are to be doing a lot of performance and cosmetic parts for the Focus. Primarily the focus because they realize that, although you could have bought a SVT Focus, people in that age group couldn't afford it. They buy new or more likely, used Focus's and then add aftermarket parts as they go. Ford realizes this is a huge profit making sector of the industry and wants to capitalize on it. So, having said that, although I have heard no specific mention of Ford doing the same for the Mustang, outside the stuff they always offered, it wouldn't surprise me in the least that they are ramping up to offer Ford aftermarket parts for the Stang. Typically, they would introduce that stuff after the first model year. It makes sense to test that aftermarket equipment while testing the Vert Prototypes. It very well could be, that you saw, not only a vert, but one with Ford aftermarket parts.

EDIT!!
After I posted this, I saw someone else posted this Ford News article about this very thing. It appears, they are doing it for Mustangs. And, on a larger scale than I suspected. http://www.fastdrive.org/news.php?extend.199