You might be able to negotiate a v8 hardtop in lieu of your reserved Mustang convertible, seeing as, to their overall customer base, an Ecoboost convertible may be "more desirable".
USTAR "Mustang Convertible or similar" = Mustang Convertible (turbo 4cyl), Camaro 1LT Convertible (turbo 4cyl, no more v6's), Mercedes C300 Convertible (turbo 4cyl), rare Buick Cascada Convertible.
RTAR "Camaro SS convertible or similar" = Mustang GT convertible. No more Camaro SS's... hardtop or convertibles. Last were MY2017.
Who knows though, maybe they'll get in some 2019 Camaros by the time your reservation comes around.