Validation guidance for when exceptions preclude using uniqueness constraints

Summary:

In a Universal application viewpoint with one node type but multiple alternate hierarchies, a specific property is required to be unique across all members of any individual hierarchy — but with the single exception that large numbers of nodes are required to use the value "NA". Since that will obviously preclude using a formal constraint, I'm wrestling with whether there's any possible way to fall back on validations to enforce the non-NA uniqueness requirement.

Content (please ensure you mask any confidential information):

Exempting the "NA" usage is a non-issue for a validation; it's building a viable Expression Builder statement for the