Hmmm? If the film was forced into a spherical shape rather than a semi-circular shape. The film would all be the same distance from the pinhole and you could make the focal length as short as was practical with the equipment you had. It would also give you a fisheye view, I think? I imagine the challenge is getting the film into this shape.
I agree with Vaughn: I have a Zero Image 4x5 camera and at the shortest "focal length" of approximately 25mm (from pinhole to film) the light drop off at the corners is significant. Vignetting on a B&W image is a nice effect but it doesn't look as good with colour film... IMHO...