Not that complex if we want a beer at the end of it!
If we chose the right subject we could tell where the exact point of focus was and take that into account if there was a comparative difference between cameras. Also, shooting at f/8 and f/11 would help even things out a bit too. As for differences between individual cameras, Simon and Fraser both have a Rolleicord Vb each, and I have two Yashinon lensed Yashica 635s if we want to even individual differences out a bit more and look for general characteristics.
It would be interesting to do a reasonably comparable test but I won't lose any sleep over it if it doesn't happen.
If I were doing a direct comparison between cameras then the last thing I'd do is put a close-up converter in front of the cameras' lenses... unless I wanted to see how the cameras compared when using a converter. I think a straight shoot-out using the same subject, angle, film, shutter speed and aperture, and firing the shutter at the same time to capture as near to the same lighting conditions as possible would provide more comparable results?
However, life is short and so is availability of spare time, so this may well never happen!