In my app I have a requirement for a functional area to open in an overlay such that it sits on top of the current route which could be almost any other route in the app.
<I think you got confused by open in overlay requirement and you are trying to solve non-existing problem.
Opening messaging functionality in another page in your app or opening it as overlay on top of the app is just a design thing that should be handled by CSS and existing app router. Just create messaging route, controller, etc.
You didn't say that, but in case you want to boot separate application there, the best approach is to open it in the iframe. New app, new lifecycle, etc. Again, how you display it inside another app is just matter of CSS.