Not sure what you are after here.
the 720P file is in fact 720 X 960.
Thats what it is suppsoed to be.
When you look at it in FCP, it gets stretched horizontally to 720 X 1280.
TV stations and DVD players alike all stretch the footage back to 720 X 1280 when you view it but the actual file remains the same.
The same theory goes for 16:9 NTSC or PAL footage.
Letterbox is someting different.
The black bars in the top and bottom you get after fitting a widescreen image into a square box is called letterbox. I dont understand why you would want to do that?
Johan Polhem
Motion Graphics
www.johanpolhem.com