The difference with having support using the Responders solution is that they are per route. The suggested feature would be "global", for all routes, as I see it.

Perhaps we can achieve this by enabling route endpoints to fire an event like we did for the TODO app instead:

@Inject
Event<ResponseHeaders> headers; 

public HttpResponse login(final AeroGearUser aeroGearUser) {
    performLogin(aeroGearUser);
    fireResponseHeaderEvent();
    return createResponse(aeroGearUser);
}

private void fireResponseHeaderEvent() {
    headers.fire(new ResponseHeaders(AUTH_TOKEN, token.get().toString()));
}

The endpoint method login above will fire a response header event with the auth token for the logged in user. In the TODO app, the CustomMediaTypeResponder @Observes this event and will set those header of the HttpServletResponse.
How about we move the ResponseHeaders (maybe call it something different) to the controller and build in the @Observer method into AbstractRestResponder. This way the "only" thing a developer has to do is fire the events with the header names and values that he/she wants to have set.

ResponseHeaders (which should really be have methods for adding more headers I just realized..doh) looks like this:

public class ResponseHeaders {
    
    private Map<String, String> headers = new HashMap<String, String>();
    
    public ResponseHeaders(final String name, final String value) {
        headers.put(name, value);
    }
    
    public Map<String, String> getHeaders() {
        return Collections.unmodifiableMap(headers);
    }
}