Resilvering may not be possible from Mamiya... I would suggest you contact some amateur telescope builders and see if they will recoat the prism for you. I'll tell you that opening those prisms is not the easiest thing in the world, so you might be better off finding a good used meter/prism and replace yours.
There might also be some lens repair companies that can do this, the only one I can think of is Arax Photo somewhere in Russia, might be worth an email to them. From their feedback page
http://www.araxphoto.com/feedbacks/2.shtml
I sent Carl Zeiss and Schneider lens elements to ARAX for polish and coating service. These were once high-quality planar and equivalent elements but coatings and glass surfaces were in very poor condition. Without any hesitation -- I recommend the service. The results are *outstanding*. Results of film tests post-servicing with the lenses wide open and stopped down to their sweet spot are incredible. I will definitely send glass in future when the occasion arises.
Phil A. Wed, 17 Oct 2007
Seattle, USA
You may need to pull the prism out of the finder and strip it to bare glass for them to recoat it. I have found few places that still list that they will recoat lenses, and fewer still that will reply to an email... Find the telescope makers and see if they will take the project on as a personal challenge, are do the research and try it yourself. What's the worst that can happen? You ruin an already ruined finder.