Django changing the default object manager
The default manager is attached in the function ensure_default_manager in django.db.models.manager. It attaches by default a manager of class Manager. You could monkeypatch this function to attach a different (subclass of) Manager.
But you have to consider whether this is the most ideal solution to the problem you're trying to solve.
Is there a way to change the default object manager for all Models? (which would include the object managers on third party apps)