I didn't realize that was just a lightbar. What's the point of that? No wonder nobody gets them.
Anyways, I agree, the Mustang needs some type of rollover protection system behind the rear passenger. I would say they could use a pop-up rollbar like used in the C70 and BMW convertibles (like 3-,6-series, not Z4) (see picture below), but it might be too expensive a system for a Mustang, since Ford tries to keep the price down on them. Maybe that system could be used on higher models, while twin or single hoops could be standard?
See the black things behind the rear headrests, those are the pop-up rollover bars.