Adding a Unity 3D view on top of UIKit again

I know that this could've been asked many times before. But, I would still like to explore the possibility of loading a Unity 3D view to a running app that was built without using Unity.

In this case I would be displaying 3D objects on top of a camera view, and I'm using Unity for this. The camera part is already handled by another 3rd party library.

There's a callback in every generated Unity Xcode project:

 int OpenEAGL_UnityCallback(UIWindow** window, int* screenWidth, int* screenHeight,  int* openglesVersion)

I would like to use this window and assign my own view on it.

What I don't understand is how to start unity "on-demand"? Such as when we're loading a view of a UIViewController, not when the application is launched.

Could I just use the "StartUnity()" setup that was usually being called on application:didFinishLaunchingWithOptions: ?

Then, I could read this callback, and add the view controler's view into this window. This window would be the app delegate's window.

Has anyone done this before?