You'll need a file edit for this. Not sure if this can be made in to a plugin, I didn't get the time to look at that, you can request for it in the requests section to see if it can be made in to a plugin.
Here is the edit you need to make:
File: inc/functions_forumlist.php
Find: (Around Line #315 [#276 for v1.4])
Replace with:
If you also want to format the mod list when viewing the forum, then you also need to edit forumdisplay.php
File: forumdisplay.php
Find: (Around Line #200 [No need for this edit in v1.4])
Replace with:
Here is the edit you need to make:
File: inc/functions_forumlist.php
Find: (Around Line #315 [#276 for v1.4])
$moderators .= "{$comma}<a href=\"".get_profile_link($moderator['id'])."\">".htmlspecialchars_uni($moderator['username'])."</a>";
Replace with:
$moderators .= "{$comma}<a href=\"".get_profile_link($moderator['id'])."\">".format_name(htmlspecialchars_uni($moderator['username']),$moderator['usergroup'],$moderator['displaygroup'])."</a>";
If you also want to format the mod list when viewing the forum, then you also need to edit forumdisplay.php
File: forumdisplay.php
Find: (Around Line #200 [No need for this edit in v1.4])
$moderators .= "{$comma}<a href=\"".get_profile_link($moderator['id'])."\">".htmlspecialchars_uni($moderator['username'])."</a>";
Replace with:
$moderators .= "{$comma}<a href=\"".get_profile_link($moderator['id'])."\">".format_name(htmlspecialchars_uni($moderator['username']), $moderator['usergroup'], $moderator['displaygroup'])."</a>";