crop or offset the camera display?

I can't find an easy way to explain what I want to do. For the sake of argument, I'm using fish-eye to describe a camera that has a very large FOV, large enough to distort the image 'uncomfortably'.

I want a fish-eye camera lens that puts the center of the fish-eye, the least distorted area of the lens, near the bottom of the display frame. So let's say I want a 800x600 result. If I had a fish-eye lens that rendered a 1200x900 frame, then I could 'crop' the center top 800x600 portion to get the effect I want, trimming the edges off of the left,right, and bottom. This would effectively move the 'focus' of the camera to the lower portion of the screen, and only fish-eye distort the top, left, and right areas of the display. Is there any way to 'crop' the display, without actually draw-processing the trimmed areas?