I know the original question is old but there are many of us still out there trying to accomplish mocking in Windows Store Apps.
I have recently started to use MoqaLate which states: Mocking for Windows Phone & Windows Store apps.
It actually physically creates the MockingClasses after you Build your solution.
So, after build you will have a folder with some Mock implementation of your classes.
I think is not the best option. Run-time mocking will be the ideal but in the meantime it creates the mocks for me.