Rob:
It is not the film format that matters, it is the angle of view of the lens. A 50mm lens on a 35mm camera will see almost exactly the same scene as a 90mm lens on the Mamiya 7 (ignoring the fact that the 35mm format is slightly more rectangular). Accordingly, if the flash covers the angle of view for the 50mm lens on the 35mm camera, it will cover the angle of view of the 90mm lens on the Mamiya 7, without any change of setting.
For flashes with adjustable coverage, you just need to know what the angle of view equivalencies are between the lenses for 35mm lengths and the lenses for the Mamiya 7 camera.
Here is a link to the Mamiya site, where the lenses are listed. If you click on each lens, you can read the applicable angle of view:
http://www.mamiya.com/mamiya-7-ii-lenses.html
If you need one, I expect I can find a link listing angles of view for common lenses for the 35mm format as well.
Matt
P.S. here is a link for a table showing the details for all the lenses in the Olympus Zuiko (35mm) line - it shows which angles of view correspond to which focal lengths:
http://olympus.dementia.org/eSIF/om-sif/lensgroup/lenstable.htm
P.P.S. All of the above applies only to cameras and lenses without movements.