The only guy that comes to mind for me is Ban Tang. I know he used to do sharpening as well as making his own blades, but if it's steel I'm sure this guy can make is screaming sharp. Pretty sure he's located in California. You could contact him and see if he'd sharpen it.
Real katana? I'd look for someone who specializes in sharpening them, might not be easy to find but you don't want someone sharpening a "very expensive katana" with anything other than a wet stone if it's a true Jap hand forged blade