I have thought about such a field, too. Sometimes the plate is better visible while watching the film than on the snapshot, esp. when car or camera are moving. Sometimes it becomes unreadable when reducing the picture or file size. The contributor could enter the correct registration in the field then.
It could be difficult in the case of a faked plate, when the real plate is known. Or when the car has different plates during the movie (www.imcdb.org/vehicle_226445.html). Which one should be recorded in this case?
All in all I think this could be a nice feature, but I doubt that it will be accepted by most contributors (like the screentime or the foreign movie title fields, which often stay empty).