and I would estimate that he (most likely) knows what he's talking about. But you are right, we do need to be skeptical & see if it can be confirmed by authentic Seiko sources - unfortunately, I don't have any on hand :>(
But - your explanation of the plating process does raise the possibility of a production lapse. But - like you hint, Seiko certainly could have taken advantage of the fact to easily & cost effectively make special looking movements for internal purposes.
As far as monetary considerations - if it is indeed true the copper color movements were made in exceptionally small numbers: rarity factor!! It drives a large part of the collectibles market!! After all, why else bother to advertise your item as "RARE!!", "Very RARE", "Extremely RARE", and other adjectives in auction & bulletin board listings??? :>) BTW - I've seen SO MANY "rare" labeled watches on eBay.ph - usually things like fairly common models of Seiko auto chronographs that are regularly offered for sale - often the truly "rare" / hard-to-find ones don't BOTHER to say "rare". The real collector KNOW it already! :>)
The Patron Saint of Seiko Collectors