We tried this with our fleet of landrovers. Each one has a waterproof interior and is plumbed with sprinkler pipe with hoze-lock connectors on the back of the vehicle so it can be plugged in to the water supply.

It doesn't work, the water spray is not strong enough, or directed enough to remove the mud and sand. We still have to have people going round with water jets to spot clean. A have my doubts a fully automatic system could ever work effectively.