I once had an Opemus 5 and now use an Opemus 6 for colour. I was wondering why they had increased the diameter of the concave/convex mounting plate between the models and this problem with the Rodagon, and 'fat' lenses in general, might be the reason.
Is the problem with the stop-down lever, or with the actual lens-body itself? I have no idea if your particular Rodagon has the pre-set aperture lever or not, but it could be the widest part - it could also be far enough away from the mounting ring that the width isn't a problem. Perhaps the stop-down lever can be removed, if that's the bit which is too wide.