Yes, you can create a class extending MovieClip set it as the Document Root class and initialize the Facade from there. Or could instantiate it on a MovieClip instance on the Stage if the movie is going to be timeline based.
Your major concern will be in how to prepare the View.
In an MXML application, the Application object takes care of building all the initial View Components, and they're alive and waiting to have Mediators connected to them by the time you send a Notification (on creationComplete) to prepare the Model and the View.
This needs to happen differently in Flash. I will put a example online soon, I promise! -=Cliff>