In Magento, there is only one scope for Customers. A customer is either unique per website or globally. Because of this, there is only 1 value for each customer per attribute. This is different from products which can have store and website level values.
I suggest that you either change your customer scope to website, if that is what you are actually after, or, more likely, come up with a different way to store the data you are trying to store on the customer entity.
You might consider creating a new model and structure it much like the static block model. This model has an identifier which can be the same across different instances. Each instance is tied to one or more store views. You could then store the "identifier" value for your model in the new customer "validation_rule_identifier" that replaces the "validation_rules" attribute that you have now. When you need the logic that was formerly using the validation_rules attribute, you should load the new model and filter by the current store and the current customer's validation_rule_identifier value. This should allow you to accomplish what you are hoping to do right now.
Unfortunately, the solution is not simpler although most of the logic is already built inside the static block model classes.