If you're offering a boutique development service, then any "replenished" product would be pointless as you wouldn't do enough volume of it to make it worthwhile or effective. I'd go practice on a bunch of your own film with Pyrocat HD or some similar pyro formula so you can have ONE alternative that is sufficiently different from your standard b/w process. As others have noted, pyro is the one "exotic" developer that more people would have heard of, and that they might not be willing to try themselves at home out of fear from its reputation. The other way to approach it would be to offer boutique developing, but the customer supplies the chemistry AND specific development regime instructions. If both are not provided, you won't run their film. But that would have to be a VERY expensive offering.