We have many ways to search for event frames, but I do not believe we have a way to search if an extended property does not exist.
So, you seem to have a custom program that manipulates event frames and creates extended properties the moment it deals with a particular event frame.
Is possible to simply have all event frames created directly with the extended property set to some default value. If so, you can search using in the following method and specify that you are looking for that default value.
The search I need must be used to verify that I have migrated all my event frames to have the extended property. Therefore, I cannot use the approach you suggest. I will however as you suggest that all event frames will at least have the default value in the extended property.
I found a way through by going directly on the PIFD database. I can do that as it is a one-time consistency check and not running production code.