As for your in_array question, you want something like:
Code:
<xen:if is="in_array({$user.user_id}, array(X,Y,Z))">
<li class="ribbonAlienInvasion">
<div class="left"></div>
<div class="right"></div>
Alien Invasion
</li>
</xen:if>
Replace X,Y,Z with a comma-separated list of the user IDs of the users you want to have this ribbon.