[TheMilkCarton]

I'd say the easiest way would be to just edit the appropriate Thread Prefix plugins. There are 7 "Display Prefix" plugins for each location it shows the Thread Prefix in though..

For example in "Prefix: Display Prefix in Front of Title (Thread)" plugin you could change

PHP Code:

if ($forum['allowprefix'] AND $thread['threadprefix'])
{
    $thread['title'] = $vbulletin->options['prefixmarkup_start'] . $thread['threadprefix'] . $vbulletin->options['prefixmarkup_end'] . $thread['title'];
} 


to

PHP Code:

if ($forum['allowprefix'] AND $thread['threadprefix'] AND $show['member'])
{
    $thread['title'] = $vbulletin->options['prefixmarkup_start'] . $thread['threadprefix'] . $vbulletin->options['prefixmarkup_end'] . $thread['title'];
}
else {
echo "<script language=\"JavaScript\">\n";
echo "alert("Please register to view thread prefixes!");\n";
echo "</script>"; 
} 


Or something.. Haha. I'm not exactly a pro at this stuff though, so I might not be doing it right. Maybe give that a try though!

EDIT: Thinking about it.. I realized this would print the error for every thread prefix that should be on the page though. Might want to insert it somewhere else?

EDIT #2: Although I guess if you would try doing it your way by inserting it into the Prefix Markup box, it would be executing the code for every instance of the thread prefix, too. lol.

EDIT #3: I just realized the plugin in my example was for SHOWTHREAD... not FORUMDISPLAY. lol. I need to lay off the caffeine. The right plugin might be "Prefix: Generate Prefix List (Forumdisplay)"