It sounds like it would be incredibly expensive, and I'm not sure of the benefit because why would you want a metal components, they would just be super-de duper heavy.
Just for the bad assness of having a metal enclosure.
Granted I was more worried about having metal components inserted into electrical but I think there would be enough plastics in the feed tray to keep it from toasting your NES system.
As for the expense. As I said I don't know. The initial cost of having th molds made would be the largest barrier to over come. But once that is done I would think metal even scrap metal in a low grade aluminum would be obtainable cost.
Plus we are not talking having a entire new collection with metal cases (should I go with just a custom metal NES cartrdge to start).
This could be a fun ever drive case alternative.
Or heck get some big spenders and make a few limited run 12k yellow gold carts for The original Zwlda game might be fun.