I have not heard of any 7X17, 8X20 or 12X20 Chamonix cameras with reversible back. No doubt they could make one custom for you, but this would add a lot of size to a banquet camera. A reversible back is more realistic for the 11X14, 16X20, 14X17 and 20X24 formats.
My 7X17 Chamonix is beautiful and well made. It is fairly light so hanging it on its side is feasible. But working with the camera on its side for vertical compositions presents a number of complications. It is OK for a shot now and then, but frankly if I planned to use a banquet camera a lot for vertical shots I think you would be much better off with one of Richard Ritter's cameras.