This is a poor approach because it's divorcing the data validation from the data model.

Actually its not possible to correctly check metadata on a content item because its generated in the modules (at least right now it is). So it would not be easy to implement this well.

But even if we did, it makes much more sense just to devise a better data model, with validation rules, rather than trying to handle this from a distance. Simpler for the user, and simpler to implement.

I'll add some new issues that make more sense.