No, models are shareable piece of code, and normally having those pieces pulling data from a particular app-specific structure is bad. Instead, we recommend pushing data/config into the models.
In other words, you can do something like:
var foo = ac.models.get('foo');
where "someData" is something that you can put together in your dispatcher.
That being said, if the model has a "init" method, a config structure, with the mojit's configuration will be passed into it at the moment you access it thru ac.models.get``, and this configuration is a combination of defaults.json (from the mojit itself) and the config fromspecs` in application.json for the mojit's instance.
As I mentioned in another reply for one of your questions, you have to mock it :)
I will recommend you to look at some of the tests under yahoo/mojito/tests/units/, to see how to mock "ac". Any data used, or required, by any YUI module that you have in your application, you should mock it when it comes to tests.