I saw a show here at HCP last summer by Lyle Gomes, who shoots 4x5 with a mask on the groundglass and uses a cut-out darkslide that turns it into a panoramic negative of 2x5. He enlarges these on conventional enlarging paper and achieves some very good results. Of course, this is not nearly as fun as hauling around a 7x17, but might be more sane in practice.
I tried the 2x5 trick and it works really well. Being tool challenged I bought the modified darkslide from Bender Photographic. It costs around $20.00.
I have a couple of old 5x7 film holders that are in bad shape so I might try my luck in in making a mask for 5x7. That will give two 2 1/2 x17cm (well almost) pano negs on one sheet.
Like Dan, I use the whole negative but put a piece of stiff black cardstock on the ground glass that is cut out to the proper dimension to use as a compositional tool.