Property Validation and Generation (PVG) policies not enforced on pre-Active Roles creation objects with blank attributes?

PVG is working perfectly for objects created in the Active Roles web console, and we've never had a need to enforce property validation on our many, many objects that were created before we deployed Active Roles.

Until now. 

In our policy rule, we have the "<Property> must be specified" box checked, which prevents objects from being created with a blank attribute value, or from the attribute from being blanked out in the web console. 

It does not, however, enforce existing objects from remaining blank if an admin modifies another attribute with the web console. Ideally, if an admin modifies any other property in the web console, we'd want a "policy violation" error banner to appear, preventing them from proceeding until the property has a valid value entered. 

Is there an elegant way to accomplish this?