Remco makes a lube pump kit for many automatic transmission vehicles.
Some of them are convertibles.
My PT Cruiser Turbo convertible can be towed four down using a lube pump. In fact, there is an old one installed in the car. I have never used it, and don't plan to. I don't even know if it is any good! My Demco dolly works just fine for taking the PT with us, I see no need to spend the money for a towbar and brackets for the car.