Currently it takes a couple of steps.

First in the send_header function in libs/ubbthreads.inc.php you'll see a big list of global variables. You need to add $wrappers to that list of globals.

Your header file would need to be done in php, but you could then use the $tbopen and $tbclose variables in your header.

Code
<?php
$tbopen = $wrappers[$wrapper_id]['open'];
$tbclose = $wrappers[$wrapper_id]['close'];
?>