Occluding portions of the display

[westbyb]

So my goal is to be able to cover portions of the display (i.e. cover the left/right side of the screen, shrink the size of the display, etc), but I don't want to go about using an object and just placing it in front of the main viewpoint.

Nothing I've found seems to be able to do it so far, so I was wondering if you guys had any suggestions.

Thanks!