XML Document and property promotion

I had a custom code which returned XMLDocument(xDoc). In my orchestration, I created a message of type XLANG.BaseType.Any (msgAny)
Did following -
msgAny=xDoc;

I wanted message type to be promoted . I used trick of correlation on BTS.MessageType.
But it failed :( :(

After digging at all wrong places, I came to know that msgAny, when assigned value of xml document, it is treated as XDoc and not as XLANG type.
I developed a custom pipeline component and invoked the custom pipeline in my orchestration.
:) :)
And it worked :)