How can I use http request headers for content negotiation in a Mashaller?
Custom Marshallers section mentions Marshaller.oneOf overloaded methods, that seems to be what you want:
Helper for creating a "super-marshaller" from a number of "sub-marshallers". Content-negotiation determines, which "sub-marshaller" eventually gets to do the job.
The Marshaller companion object has many methods that receive a Seq[HttpHeader]. You can look into their implementations as well.
I don't have the time to look into the source code myself, but if this is not enough to put you on the right path, let me know.
Edit:
How about?
get { optionalHeaderValueByName("JFORMAT") { format => complete { format match { case Some(f) => "Complete with json4s" case _ => "Complete with trueaccord" } } }}