I had hoped to make something that would work within an "existing" content negotiation scheme, but maybe that's a bit of my own naiveté about JAX-RS. The current branch does work fine without any @Produces annotation. 

If we need to add @Produces annotations, we end up with one class per format (XML, JSON, etc). Not that bad with subclassing but then we are back to having a base class and some really thin "*Json" and "*Xml" wrapper classes -- is this the right approach? Happy to do so but would really appreciate your advice here.