XML are not meant to be viewed by the public, but by developers or tools.
Carrying author information is either part of the data (as in blog articles having an author) or it doesn't.
You can however use XML comments for that. Or you can define your own XSD and structure everything like this:
<?xml?>
<!--
Copyright notices here
From you regarding the XML itself
From your client regarding the XML contents
-->
<root xmlns:copyright="http://www.w3.org/1999/xhtml">
<!-- per file meta-data here -->
<metadata>
<!-- authors make it -->
<author name="XXXXX" />
<!-- copyright holders buy it from authors and sell it -->
<copyright name="XXXXX" />
<license type="GPL">
Lorem ipsum dolor sit amet...
</license>
</metadata>
<data>
<entry>
<!-- per entry meta-data here -->
<metadata>
<author name="XXXXX" />
</metadata>
<contents>
</contents>
</entry>
</data>
</root>
Also if it is needed by third party applications (to be extracted and shown) it would be more useful if you used XML elements not comments, as they are easy to manipulate.
I mean it's ok to have a comment with a simple text but if you want to put complex info there like I showed in my example, with sub-fields like author/copyright/license, it is easy for third party programmers to get those Node objects.
As a variation if you're feeling particularly namespacey:
<?xml?>
<!--
Copyright notices here
From you regarding the XML itself
From your client regarding the XML contents
-->
<root xmlns:copyright="http://www.w3.org/1999/xhtml">
<!-- per file meta-data here -->
<copyright:info>
<!-- authors make it -->
<copyright:author name="XXXXX" />
<!-- copyright holders buy it from authors and sell it -->
<copyright:holder name="XXXXX" />
<copyright:license type="GPL">
Lorem ipsum dolor sit amet...
</copyright:license>
</copyright:info>
<data>
<entry copyright:author="author name" copyright:holder="holder name">
<contents>
</contents>
</entry>
</data>
</root>
With namespaces the third party programmers will have an easy time separating the two layers of information.