For what it's worth, from my live chat with a Chevrolet marketing representative this morning:
Question: Is Chevrolet planning on producing a ZL1 convertible?
Nancy: The ZL1 Camaro Coupe is the model that is planned. There is no indication that a Convertible will be produced.
Current Chevrolets: 1969 Camaro Z/28 -- 1969 427/390 Corvette Convertible -- 1970 350/300 Corvette Convertible -- 2013 Camaro ZL1 Convertible -- CRT / Exposed CF Weave Hood Insert / Suede Package / Polished Wheels / MN6