You could have such a thing, but it would be heavy and mechanical. The Linhof rangefinder kind of works like that, as does the Speedgraphic: bigger rangefinder on the side, with arms and a cam that go under the lens. I think in the case of a camera like the Mamiya it would be too much bulk and extra mechanics and weight. Might as well get a Linhof and a tripod. BUT, I do wonder sometimes why the two little RF windows on the Mamiya are not further apart as it is. They certainly could be. Maybe it wouldn't make a big enough diff with the longer lenses? I don't know.