I worked on the case for a couple of my featherweights. First I cleaned them throughly, one of them just ran yellow with old cigarette smoke. ( I think I just used spray cleaner like 409, plue an upolstery cleaner called "Tuff Stuff" ) The peeling fabric I re-glued with wood glue. It's been several years and it hasn't come back up again. Then I went over the whole case with a liquid black shoe polish. The one I still have still looks really good.
I have some other cases to clean. Another black FW case, a 301 grasscloth, and one that is 50s vintage that is maroon and tan-maybe grasscloth on the bottom. The last one holds my grandmother's 66 and looks really bad. But, like Joe, I would rather save them than buy a plastic case.
I do have a couple that are in plastic cases. I've been wanting to try the Retrobrite treatment (peroxide recipe devoloped for old yellowed computers) to remove the yellowing. Has anyone ever done this:
http://retr0bright.wikispaces.com/