Neither copper or brass are good plating options for a fuel tank. First you would need to have the tank stripped to bare metal, all imperfections worked out without any type of filler as stated in a previous post. I've been told by every plating company I've talked to (20+) that there is really no way to block the openings and keep the plating out of the interior. Copper is not appropriate for fuel. I'd be intrested if you find someone that says they can do it. I'm planning on nickle plating some jugs and am told that I'll have to have the cylinders machined afterwards.
As far as powdercoating, I've seen quite a few really good ones, but I wouldn't be surprised if you struggle to find a coater that is willing to put a container that has held fuel inside an oven to bake the coating :/ just my opinion and what I've been told. Definitely subscribed and will be watching.