The MP Rest Client 1.2 TCK assumes the ${subject} should be possible but I'm getting "RESTEASY003880: Unable to find contextual data of type: javax.ws.rs.core.HttpHeaders" when I try.
I think it's possible to add an implementation of HttpHeaders to ResteasyProviderFactory.getContextDataMap(), and then make the headers from it populated to the request with a filter but is there a better way? Maybe some add-on that would make it work out of the box? ;)