Final Cut Pro 7: Zoom without using “scale”
I do this all the time. It's super easy:


Set up the 'shape' of your frame with the crop tool. 
Apply the effect 'Basic 3D' (Effects > Video Filters > Perspective > Basic 3D).
In the controls for that effect, use the 'Scale' parameter to zoom your image. It will not affect your crop marks!


Good luck!