Such an effort can be done, but it would require reverse-engineering the electronics of the Contax 645, building an electronic interface to match the Mamiya electronics, AND machining/trimming the rear mount of the Contax lens to add a Mamiya bayonet that would retain infinity focus of the lens.
Not necessarily. Although it's a rather clunky process, you
can adjust the aperture of the Contax lenses by mounting them on a Contax 645 body, adjust the aperture to the setting you desire, then hold down the DoF preview button while you dismount the lens from the body. The aperture will remain wherever you set it and you can then install the lens on another body while being careful not to move the focus ring.
I have successfully done this to use my Contax 645 lenses on my Toyo 23G view camera via a mount adapter that I cobbled together from an extension tube and a tube of two-part epoxy, so if you can successfully address the mechanical aspects of mounting a Contax lens on a Mamiya (which, as David pointed out, won't be easy), you won't be stuck shooting everything wide open.
