Don’t limit your Backbone apps to Backbone constructs. If you find that your app needs some functionality that doesn’t fit well into one of Backbone’s constructs, don’t shoehorn it into one just for the sake of keeping everything in Backbone.
I’ve found this backbone boilerplate to be helpful in this respect. It sets up modules for you and provides you with an app object that extends Backbone.Events (like in the previously-linked article). This app object can be used to store instantiated models, views and controllers/routers, and you should feel free to add your own non-backbone modules to the app object, to take care of responsibilities that don’t fit perfectly into one of the Backbone constructs.