I still think the best solution would be to handle it exactly the way HOI does unit names - no popups, and you can text-edit them whenever you like, if you like. Or not.
One additional argument in favor of this - some rulers "ruled" under a different "official name" than they had growing up. (Especially, a few of those French King Louises). This way, it could be changed when they came to power.
Also, we could add, delete, or change sobriquets, such as changing "William of Normandy" to "William the Conqueror", or something....