Well, my solution is quite dirty and hardcoded. On post inserted event onCommentAutoPublishing triggered. Each event handler return true if message is published otherwise false. Message autopublished if no false values was returned by plugins.

Plugin parameters:
[size=1]Just small amount of context data passed. Maybe we should pass $this reference instead?[/size]
[ol][li]comment (BB-tagged text message)[/li][li]ip[/li][li]email[/li][li]website[/li][/ol]
Plugin result:
[ul][li]true (message published)[/li][li]false (message is not published)[/li][/ul]

diff --git a/joscomment/comment.class.php b/joscomment/comment.class.php

index bb04ded..d98a760 100644

--- a/joscomment/comment.class.php

+++ b/joscomment/comment.class.php

@@ -976,6 +976,16 @@ class JOSC_board extends JOSC_visual {

         return JOSC_utils::censorText($text,$this->_censorship_enable,$this->_censorship_words,$this->_censorship_case_sensitive);

     }

     

+    private function _isPublished($ip, $email, $website, $comment)

+    {

+        $dispatcher = JDispatcher::getInstance();

+        $result = $dispatcher->trigger('onCommentAutoPublishing', array('comment' => $comment, 'ip' => $ip, 'email' => $email, 'website' => $website));

+        $custom_filters = true;

+        foreach ($result as $row) {

+            $custom_filters = $custom_filters && $row;

+        }

+        return ($this->_autopublish && $custom_filters) || JOSC_utils::isModerator($this->_moderator);

+    }

     function insertNewPost($ajax = false)

     {

         $ip = $_SERVER['REMOTE_ADDR'];

@@ -997,7 +1007,7 @@ class JOSC_board extends JOSC_visual {

         $title = $this->censorText(JOSC_utils::mysql_escape_string(strip_tags($this->_ttitle)));

         $comment = $this->censorText(JOSC_utils::mysql_escape_string(strip_tags($this->_tcomment)));

         if (!$comment) $comment = _JOOMLACOMMENT_EMPTYCOMMENT;

-        $published = $this->_autopublish || JOSC_utils::isModerator($this->_moderator);

+        $published = $this->_isPublished($ip, $email, $website, $comment);

         $parent_id = $this->_parent_id;

         $database->SetQuery("

             INSERT INTO jos_comment

And here sample plugin, discontinuing comments with hyperlinks inside:

defined( '_JEXEC' ) or die( 'Restricted access' );

jimport('joomla.plugin.plugin');



class plgSystemJocom_Nolink extends JPlugin

{

        public function onCommentAutoPublishing($comment, $ip, $email, $website)

        {

                return stripos($comment, '[url') === false;

        }       

}