Issue:
Customizing the file
includes/languages/english/meta_tags.php

and modifying the TITLE define works fine in the catalog but is not
reflected in the Admin section.

Root Cause:
In admin/includes/languages/english.php, $template_dir not set

This results in the defaults from includes/languages/english being used, rather than any customizations.

The workaround is to change admin/includes/languages/english.php just after

// removed for meta tags
// page title
//define('TITLE', 'Zen Cart');

// include template specific meta tags defines

to include the code

// removed for meta tags
// page title
//define('TITLE', 'Zen Cart');

// include template specific meta tags defines
$template_query = $db->Execute("select template_dir
from " . TABLE_TEMPLATE_SELECT .
" where template_language = '0'");
$template_dir = $template_query->fields['template_dir'];

This is just a workaround; I'm sure the final fix will be quite different.
Be sure to back up admin/includes/english/english.php so you can restore it
when a proper fix is provided.

Scott