Using the Title in Navigation Bar for a page status

I'm designing an app on iOS & Android. My client wants the title in the Navigation Bar/App Bar to be a status rather than a page title. Instead of "Registration" he wants to switch between "Registration closed" and "Register now". I'm having my doubts.

Would you allow it? It seems that space is meant for the title only, but in no guidelines it says specifically this is ok or not. But I feel any status message is better set in the content area, where it also can be highlighted properly. And I assume users don't really pay much attention to those titles at first.

What are your thoughts on this? Any good arguments for or against are appreciated. Thanks!