Best you can get is a 645back on a Hy6 or an RB67, no native 645s have ever rotated (as far as I am aware).
But then, unless you're really really pressed for film/cash, you may as well just shoot the full 6x6 or 6x7 and crop.

You may have gotten confused by reading about a Leaf Digital Back, some of them that fit on 6x6 (like Hassy V mount) can rotate around.