I have searched for using multiple db’s in an app, and I remember seeing how to switch db’s in the models _init() function. What I’m getting at is that I remember seeing code to getModule(), getDb(), and other ways getting information about the app. If you don’t specifically set the db connection in the Ext, you may have to using the get/setDb() stuff.
One other thought may be that you have to load the models in your Ext. I am not sure how isolated an extesnion is from the app.