I wouldn't be suprised if the Convertible was in the works while they were introducing the Camaro in Warren in August 
I have no doubt the platform was designed with the convertible in mind, same way it was for the C5 CorvetteIf the car looks as good with top up as it does in coupe form, I would bet they were designed at the same time, and finished at nearly the same time. I think they held on to the convertable to keep the hype up for another year until they can show us a production model.