Wrapping outgoing message in a soap envelope - update
I've just realised it might not be obvious what "system soap envelope schema" means so here it is:
BTS.soap_envelope_1__1+Envelope, Microsoft.BizTalk.GlobalPropertySchemas, Version=220.127.116.11, Culture=neutral, PublicKeyToken=31bf3856ad364e35
It is also worth remembering that using the XmlAssembler you're required to have the schema of the document you're outputting deployed to the server as the assemlber will look it up.
Also - using the XmlAssembler has some performance implications which have to be accepted if you're to use this approach.