Hmmm....that's a good one. MNPCtech does custom case work. Seen a lot of anodizing, painting, and custom milling of parts there....http://mnpctech.com/
Bill Owen is one of the top modders I know of, so I would shoot an email and ask for some advice, or see if he's game to offer some anodizing services.
You could do a little digging for names of companies involved in customization by looking over the PDF's regarding MaxPC's Dream Machines. I know they've contracted Smooth Creations in the past, but I think that's just for custom paint. Several years ago they used a company to chrome plate an HP Blackbird chassis, or maybe it was nickel plated. Can't remember, but you could check the company they used and see if there is an electrical process in place for contract anodizing also.
Beyond that, you'd probably have to look for a mill close to your area that does work by request.