MetadataURIResolver works only in pipelines which work on a single content, because it retrieves the content from the request attributes.

I can't, for instance, generate a XML list of contents, and make a link to a image or binary metadata for each one of them in one stylesheet.

Either resolveXX() methods should have an additional generic parameter (because providing the content doesn't mean anything for the other URIResolvers), or the metadata URI could have a form with the content ID (like "defaultWebContent://xxx/metadata" or "metadata?contentId=defaultWebContent://xxx").