[Unknown553]

In your showthread template, make sure you put $replybox directly above:

<!-- restart content table from header -->
<table cellpadding="0" cellspacing="0" border="0" width="{tablewidth}" bgcolor="{pagebgcolor}" align="center">
<tr>
<td>

And to add stuff to the reply box, simply edit the showthread_replybox template. "User logged in as $bbuserinfo['username']" would, I believe, add what you asked.