Going waaaay back, the Model T Ford's transmission used planetary gear sets just like just about every automatic ever built--except, of copurse it was manually controlled. In all but the first 4,000 of the 15million T's built, the tranny was controlled by three foot pedals. One of them engaged the transission brake, but one of the pedals engaged reverse. Model T drivers will tell you that jabbing the reverse pedal was more effective than the brake in slowing the vehicle.
Since planetary gears sets are always in constant mesh, and gear changes are effected by applying clutches and brakes ( or bands) to various drum combinations, it is possible to engage reverse at any time, unlike sliding gear transmissions. However, it does place major stress on the components. Most modern auto trannys are equipped with electrical interlocks to prevent reverse from engaging while driving forward.