6 inches (about 150 mm) is only slightly "widish" on 4x5 format (I call 180mm "normal" for 4x5), so I would not expect any serious falloff.
It is not too hard to make a "center filter" dodging mask for printing pinhole pix. Expose a piece of film in the camera to an evenly-toned subject (sky, grey card, etc.). Then make a contact positive from that negative, exposed and developed to the proper density to mask ("dodge') the corners of the frame.
Clever. I like the elegance of it, especially when thinking about funky curved film planes. I've always just made a dodging tool and done it by hand.