I have found a possible solution, however I have not had a chance to test it. Here goes:
The Cokin X-Pro Filter holder can be affixed to various lenses with traditional screw-in adapter rings. In addition, Cokin has a Universal Ring (# X499) for the X-Pro holder, which uses four (4), nylon-tipped thumbscrews to clamp around the outside diameter of a unusual lens or accessory. This adapter ring will easily accommodate the size and unusual shape of the Mamiya AF 28mm D lens. This adapter ring also allows you to fit the entire filter holder assembly as close to the front element as possible. In fact, will the universal adapter ring, a good portion of the filter holder is behind the front element, as you four (4) thumbscrews are clamped to the rear of the built-in lens hood of the Mamiya 28mm lens.
If I recall, the Cokin X-Pro Glass Circular Polarizing filter is approx 120mm in diameter. This additional diameter over traditional screw-in filters, while being placed as close the front element as possible, might do the trick.
I have used this exact filter holder combination on the Canon and Nikon 14mm f/2.8 lenses, attaching it to the built-in hood with the X-Pro Universal ring, and eliminated vignetting all together. Considering these lenses have a wider angle of view than Mamiya 28mm, there might be some potential here.
As some of you might be aware, the X-Pro system is noticeably better (in regards to construction, design, precision, and filter quality) than the smaller format systems from Cokin. If you can find a dealer who has some Cokin X-Pro stock, take a look!
Once I have a chance to try this out, I will be sure to report back.
Looks like you can get away with this with the p-30 but I don't see anything working on the p-45. I would love to hear if anyone does find a solution. I really do miss a polarizer with this lens. Too bad. Thanks!
Tim Ernst in Arkansas