I know at least two "startups" doing this per city, in Brazil. Except you don't need a smartphone to call them... :-) It's amazing how the obvious in some countries is, sometimes, a novelty in others. Another example: I found out a couple of months ago that, in Chile, there's simply NO ONE offering lockers on beaches. Meaning you have to drive to the place and leave your stuff in the car, or simply not take a wallet/…
It uses a sort of mobile water "pump" - a lot faster and more economical than hand + towel. But I suppose they use something similar (if not the same piece of equipment) there