I just watched Eric's video and the method he uses is different to what I'm used to doing. Digging around that way relies on expert knowledge of what you can and can't do - it will be exceptionally easy to break a file if you add something invalid or remove a necessary node.
Sharepoint injects custom xml in a way that enables you to insert mapped Content Controls via Insert > Quick Parts > Document Property > xxx. Normally there are 15 entries to built-in properties in there but if your Sharepoint template has injected custom xml then you can pick some others which might be what you are looking for.
Are you able to post a sample document that has come out of Sharepoint so we can experiment with the particular custom XML you are working with?
Greg Maxey has done some great work on replicating the Sharepoint xml injections. That may be somewhat relevant to your question although it may just be a distraction if the properties (in para 2 above) are already in the file.
Redirecting