http://www.magplate.com/
Located in Lowell, MA

Owner's name is Steve. He is an absolute genius (not kidding, member of MENSA, B.S., M.S., & MBA) and is the only person in the world that can nickel-plate magnesium. I used to work with Steve and was wondering if there were any parts in one of my pins that would look good plated, and if I come to the conclusion of "yes", Steve will be the only person I trust plating those parts. I don't know if he's ever done things like pinball parts, but bike parts and countless other industries he's served.

Full disclosure - I also am related to one of the owners, but this isn't a shameless plug. Once you talk to Steve, you'll know he's on another level when it comes to plating knowledge.