There's a catch 22 here as:
1. You can only create the XML when you know the cid values to put in the XML
2. You only know the cid values when you marshall XML into the SOAP Message, but
3. You can't marshall the SOAP message until you have created the XML.
 
I know that you can get around this problem IF the generation of the XML and the SOAP Message is done by the same software at the same time. Although this will often be both possible and desirable it is, I think, something that will often not be possible to do.

This problem is exactly why I argued for relative URLs for attachments in SwA 1.0.  Content-location does suffer the catch-22 dilemma you outline.