More Simulacra and Simulations
The short answer: Of course.
The longer answer: Welcome to the state law morass that governs rights of publicity. In general, celebrities who have developed valuable personnae have the right to protect it from unauthorized commercial exploitation. (Everyone's favorite case on this subject, for the amusing facts if not the outcome, seems to be Vanna White v. Samsung Electronics America, 971 F.2d 1395 (9th Cir. 1992), in which Vanna sued the company over an ad with a robot representing her.)
What does that have to do with a Sim knockoff of an Oscar gown? If the virtual gown for sale is depicted on the actress who wore it to the Academy Awards, that's an interesting question.
Designers don't compete to dress celebrities out of concern that the poor girls can't dress themselves -- there are stylists for that. Rather, the free gowns, shoes, handbags (and rumored monetary compensation) are offered in the hope that the celebrity will be photographed and the image will be frequently editorialized. Money simply can't buy the kind of exposure that a Best Actress winner's dress will receive for free. So the nominees, presenters, and other beautiful people are in effect renting their celebrity status; their bodies become billboards advertising fashion houses. The ultimate idea is to draw attention to the brand and sell more dresses -- real, not virtual.
So, if we view the agreements between designers and actresses as a financial transaction, the use of an actress' image to sell a virtual gown might violate her right of publicity. After all, what if Reese Witherspoon wanted to make money by modeling virtual gowns (as a Sim, she's certainly tall and thin enough)? It's a good thing that at the moment her real world far eclipses any virtual one.