If you're in the business of selling a product along with customizations, the product needs to support customizations without having to fork the build each time they sell it.
Sounds like you've tried explaining that, to no avail. Instead, try estimating the cost of adding your "customization the right way" for one table with maintaining, say, half a dozen versions of the product with different customizations, and fixing a bug across off of them. My bet is that they will see that they're pretty soon money ahead having a unified codebase and schema. And a developer who isn't driven insane.
