Conditional DFF Validation Checks

Morning, Gurus!

Has anyone seen an elegant solution to *conditional* DFF validation?

For a simplified example, assume a custom Table-type Validation Set allows selection of "Referred By" business partners on a Sales Order form. These partners come and go and this is logged in a custom table referenced in the Validation Table Information, so the values available in the related LOV change over time. While the order is open, we want to be sure that we are only logging references from current partners. In effect, the error for an invalid selection is a reminder to update or remove the existing value. In short, all is right with the world. However, if the order is closed and we