The Media Type itself should be as liberal about what it responds to as is sensible.  Routing to that media type is still the decision of what gets integrated.  If a service wants to respond to text/hal+json I think it should be able to do that.  In fact I would argue that a service _should_ do that whenever possible.

This is one of those cases where being correct is worse than failing gracefully.  OR put another way... thi is a large scale design/architectural decision about Crichton and Representor - do we want to be strict, or do we want to fail gracefully.  HTML fails gracefully (it's in fact in the HTML spec that it should).  Do we want Crichton to be smart about this, or do we want it to be strict.
Regardless of which we choose, whether or not to return a 406 on 'text/hal+json' is a choice the integrator _should_ be able to make without opening up the serializer class - so that needs to be supported.