Is it possible to override the htmlheader.txt and textheader.txt on a per user basis?
Are you referring to generic_message_footer and generic_message_footer_html settings in the config?
Yes ... sorry ... I currently have those as follows...
$config['generic_message_footer'] = 'textHeader.txt';
$config['generic_message_footer_html'] = 'htmlHeader.txt';
... and was wondering if there was a way to override on a per-user basis.
Not built in but it could be done with a plugin.