IMO the best thing you can do to prevent scratches would be to remove the prep brushes. Next best thing might be to add instructions that the prep brush should only be used on the grill and windshield to loosen bugs - I've seen people scrub their whole car down, picking up dirt from the fender wells. A grit guard in the bottom of the bucket would also help, along with a sign saying to rinse the brush off thoroughly before use, and not to use it on muddy cars. At least that way if they scratch their car with your brush, it's their fault.
Neither hogs hair nor nylon brushes will scratch paint if the proper care is taken.