You are correct in thinking that Sharepoint adds custom XML to a document for attributes/properties configurable via the Sharepoint properties. That custom XML can be extracted in entirety (each of the custom XML files) or you can drill into those files to find a particular xml element value and extract that. It isn't a simple task to extract a spreadsheet summarising document properties because the number of custom xml files will likely vary from doc to doc and there could be somewhere between 2 and 10 of them - each one containing a variable number of nested xml elements.

In my experience, Sharepoint adds more than one xml file and the namespace and elements will most likely vary depending on how your Sharepoint library is configured.

If you have a task of updating a particular Sharepoint property on a large number of files held in a Sharepoint library, the easiest way would be to create a Table View in that library which includes that property and then use copy/paste to fill that property's column.

The alternative to this is much more time consuming:
1. Download all the files to your hard drive
2. Work out which custom xml file contains the field you want to update - you need its namespace and the xml element's unique name.
3. Create a macro to open each of the documents, update that xml element's value, save & close.
4. Upload all the docs back into Sharepoint.

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Andrew Lockton
Chrysalis Design, Melbourne Australia