The whole idea of a pre-matured filter seems like a good idea to me Tom. I have seen several threads over the years that suggested that fish could be used as a way to remove dead skin from people in some kind of symbiotic method.
Although I have no experience of such a method, if the fish are maintained in a healthy environment by having appropriate water conditions and feedings, I have no objection to people being allowed to place their feet into the tank that holds the fish. What people can expect from doing so strikes me as immaterial to the fish's welfare. As long as the fish are well cared for, why not allow people to add their feet to the tank. My own fish will pick at my fingers if I leave them in the tank after a feeding so I doubt that similar circumstances will harm fish in a commercial setting. Although it may not address the original concern, I doubt that having fish present in a basically commercial setting is itself a problem.