Thank you so far for the replies.
It is not commonly known that f.e. fresnel screens do not show the real depth of field, it's shown always much bigger and on the final picture it is much smaller. Even a ground glass screen shows a bigger dof because of the "thickness" of the ground layer, but comes much closer to the final result. I have selfmade single element lens with a quite big aperture and many aberrations and was shocked how much smaller the real dof on the pics was compared to the fresnel screen image. Sometimes the screen makes a huge difference and the better screen on first sight might be the worse choice.
After all, it could be the best solution to get a working Zenith B and pass on the mirror lock-up. The screen image is not so dark as on the early post-war cameras and it has a usable prism. And you have a weapon for self defense :-D
To get an impression of what I'm talking about here's a "real" picture. Neither microprisms nor split images make any sense with such a lens.
nighthawk at the diner by imagesfrugales, on Flickr