Sorry, you lost me a bit. Are you calling the pipeline inside your Orchestration? If so, I don’t think that will work since Message Type is demoted when you send out the message from the Orchestration.
You can always cast the message from an Xml Doc to a typed message… I did that in one of my BizTalk 2004 untyped message samples. I think it was this one: http://www.biztalkgurus.com/Samples/Untyped-Documents-Promoted-Orchestration.html
Have you tried the Xml Receive Pipeline but set the Allowed Unrecognized Message = True? I think that still promotes Message Type.