I suspect you are right, because i've done a lot of Googling with no luck. But I'd sure like to be able to use some of those Nikkor lenses on my Mamiya 645. I'm also wondering, though, if the flange to film distance would even allow such an adapter if a person could find a machinist to cobble one together. It may be to begin with that the diameter of the male end of the helical is too big to fit the lens opening of a Mamiya.
I doubt there is enough interest in either camera to make an adapter economic in the market place, so if it were possible, it would still probably be customwork.
bob