You're right, if your masterpage isn't instantiated, then there is no instance of it in the form object model and therefore cannot be accessed via the absolute SOM expression that Designer gave you when Ctrl-clicking the object. You have to access the template object model (the design-time model I like to call it).
You can use the following to access your object:
xfa.resolveNode("template.form1.pageSet.Page2.yourObject") //notice template instead of form
This is assuming your root node of form1 and your hidden masterpage name is Page2. Keep in mind you can only read the properties of the object, you can't manipulate them.
I'm not sure why you'd need to access an object that isn't instantiated yet, so I'd agree with Bibhu's solution.
Kyle