Possible or not?

[Andrew Green]

Create a new posticon group in the control panel, set all the permissions to "no", (except maybe administrator / supermods)

Add the icon you want to use for official threads, make a note of its id number

Add a plugin for newpost_complete

if(is_member_of($vbulletin->userinfo, X))
{
   $vbulletin->db->query("
   UPDATE ". TABLE_PREFIX . "thread
   SET iconid = Y
   WHERE threadid = $threadinfo[threadid]
   ");
}

X is the usergroup you want to trigger this, Y is the iconid for the official icon.

It will override the icon already in use with the official one for the thread when someone from usergroup X makes a post in it.