[kh99]

I think the thing to do would be to execute code in the function build_thread_counters() in includes/functions_databuild.php, do a check of the thread's posts and modify the iconid for the thead accordingly. Unfortunately there's no hook there to allow it to be done in a plugin. So you could either try to find hooks to cover everywhere the icon would need to change, or else edit includes/functions_databuild.php (normally you'd try to avoid that so that you don't have to deal with it when upgrading, but sometimes there's no choice).