Hi,
From what I understand, you can set up the activatable receive shape for untyped messages when exposing the orchestration as a wcf service. In the client that consumes this service, the message you pass to the service proxy’s method call should be of a type that is serializable, and I don’t think XmlDocument is such. Have you tried to set this message as a string and pass it that way?
FYI, here’s a good MSDN article that walks you through using WCF Adapters in BizTalk:
http://technet.microsoft.com/en-us/library/bb967002(BTS.10).aspx
You might have come across it, but I think this is a good resource to look back anyway.
Good luck,
Daniel.