Currently the Service is responsible for executing the async reply to when it is done invoking the component. However, there are many people who want to reply later in the flow - for instance after a web service is done generating the response XML. Currently there is no way to do so. Also, there is no way to do a reply to from a <flow> (Yes, I recognize flow is quite new, but perhaps they can use the same mechanism which is just a MessageProcessor)
This worked for CXF/Axis in Mule 2.x because they each had two components - one which invoked the WS engine and one which invoked the real component.