Rhys-Meyer is a pretty good suggestion, even though he's filled out a lot since his Gormenghast days. Thirty years ago David Bowie was a shoo-in for the part, but we seem to have gone away from the Thin White Duke's aesthetic of male beauty.
There certainly don't seem to be any A-List stars who remotely fit the physical description of Elric. Even most male models (usually the prettiest and scrawniest of men) seem too bulky and "masculine" for the role. I wonder if you could cast a woman in the role? Keira Knightly is completely flat-chested. As is Famke Janssen. Neither of them are particularly "womanly" -- they're certainly far more androgynous than any male actors I can think of. Would an audience accept them in the part?