A question in the MSDN BizTalk Forums caught my eye a few days ago. It was about wanting to set the attribute standalone="yes" in the XML declaration on an outbound message.
I have never had the need to set this attribute before, so wasn't sure of whether the map properties or any other setting could make it be written. The other responses in the thread told be that it couldn't. It was then I thought of an old post I wrote regarding using the XMLTransmit pipeline properties to write an xml-stylesheet reference to a message. This surely had to be able to write a custom XML declaration instead. And it did.
What you basically do is set the property AddXmlDeclaration to false, and then write your full custom XML declaration in the property XmlAsmProcessingInstructions. This will write out your custom string as the XML declaration. Or in reality, it will not write it as the declaration, but since we omit the real one, it will take the place of it.
Just make sure to have the other properties correctly set (the charset) or it might not match the document itself which may or may not cause havok in the receiving end.