Hi Marlo,
I don't think that this kind or requirements can be met for existing GUI transactions - you mention VA01 and MM02. We don't have the analogon of a "key down handler" in an ABAP GUI transaction.
In the web, however, this kind of "crazy" requirements is our daily bread. It would actually be a nice exercise to write a JavaScript validator for the date format as it is specified in the SU3 settings of the current user.
The only way to do such things in a GUI transaction is by using the GUI only as placeholder, providing place for a Web application internal (via an HTML control). But for such complex standard transactions like VA01 or MM01 - nope. It would mean to rewrite the UI.
Regards,
Rüdiger