You might want to post in the modifications forum and someone using phpbb might be able to help you out. I do not know the template or variable would be in phpbb. Here is an example link you place when you find the right template and variable
<div class="
somephpbbclass"><a href="http://www.reeftalk.com/gallery/showgallery.php?cat=500&ppuser=
SOMEPHPBBUSERVARIABLE">User Gallery</a></div>