Would it be possible to legislate so that multi-rotors capable of carrying an external camera or FPV equipment could only be sold on production of proof of model club membership?
Not in the US. Model clubs have no particular legal status, here, and setting up a regulatory framework for model clubs would be even harder than trying to regulate drones. Besides, just as you figure out how to regulate "multi-rotors", someone will start selling single-rotors. Or maybe ducted-fans. One thing that becomes clear with time is that the effort to classify banned items only generates new items outside the banned list.