For large items, it can make more sense to use a camera with a digital scanning back, which is what high-end digital art repro places use.
A less costly solution for occasional use might be a DSLR on the back of a view camera with a sliding back for making stitched panoramic images, using rear rise/fall or shift for capturing the other dimension. Fotodiox makes such a back in EOS and Nikon mounts that attaches to a 4x5" Graflok back, and it's under $200.