I never knew Mamiya made pinhole cameras...

Other than replacing the shutter, I think the paint idea is the best-and it is the easiest. As stated, the paint will get drawn into the holes, and after it dries the paint on the face of the leaf can be carefully removed with a cloth with a small amount of solvent on it. Some other thin opaque liquid could be used instead with the same technique, but I doubt anything would work better on pinholes than paint.