Hi chewyalf
I'm a PHP developer but what it sounds like is that you want almost a very basic CMS system which updates only one element. If this is the case then you could try simply loading a txt file in via an iFrame if you can grant the user access to FTP upload he can change the txt in this file and upload the changes each time. I've never tried this out but it sounds like it would work. Failing the iFrame (sometimes formatting issues), you could simply put in one line of code;
<!-- your html code here -->
<div class='side_message'>
<?php
echo file_get_contents('/path/to/textfile.txt');
?>
</div>
<!-- your html code here -->
Make sure that the page has either a .php extension (otherwise the code won't parse) or you've set all html/htm extensions to parse as php. (The former is easier)
Hope this helps.
Cheers
James