Until I get a chance to talk this over with Julie in more detail, I'm going to make a temporary call on this. All long as it's not bulky/potentially dangerous, metal in costumes is ok. Metal props themselves are still no no. But I'm not going to worry too much about metal used in a costume unless there's bulk issues, other materials are just more forgiving in the crowded hallways.
Edit: After a little discussion, this is going to be my stance on this. Overly bulky and potentially dangerous metal costumes will be turned away as though they were metal props. Potentially dangerous being sharp metal corners, burrs left on edges, etc. This will be up to the discretion of security.