Is there no manufacturer that makes a back that can rotate?
I don't believe it's very practical to stop shooting to rotate the back, especially if your shooting people. Plus, the viewfinder would not allow proper composing.....I would also imagine that a mechanism to allow the back to rotate would cause some focus issues as there would have to be some ply/slack in the mechanism to allow it to rotate.
If it's that much trouble for you, a square chipped back is the way to go.... but really, the new 39MP backs provide a massive file to crop as you see fit in post.
The only rotating backs that I know of (Kapture Group) are for studio cameras, like the Fuji 680 and view cameras.