While it makes life easier, you do not need wcf…
If your orchestration is exposed as a web service:
Edit the asmx project that is created. Here you will see the guts of how to add a message to biztalk using a function call! You will also have full access the the iis http data (your soap headers). You can then add this data either to the message body before calling the “add to biztalk” function, or I think there is a message header portion of the “add to biztalk” function.
If your orchestration sending to a web service:
You can implement a custom proxy rather than using a web reference. Using wsdl.exe will give you the proxy code. You can then intercept the return from the proxy call before it is written to biztalk and in doing so again access all that goodness.