The fine Silvestri Sliding Back Adapter, made to fit many different cameras and fitting Arca-Swiss 6x9 cameras in this particular application, lets you make stitched images with a digital back.
An optional plate for your particular digital back is required; currently, Silvestri makes plates for Contax 645, Hasselblad, Leaf, and Mamiya 645 AFD mounts. Any digital back made to fit these systems can be used with the Sliding Back Adapter.
Viewing from the Sliding Back Adapter is done with a ground glass and fresnel lens combination. Some photographers may be happy just using a dark-edged loupe directly on the ground glass to compose their images, but if that's not enough there is also a proprietary bellows viewer as well as a rotating monocular reflex viewer available.