Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 18589)
+++ wp-includes/comment.php	(working copy)
@@ -1807,6 +1807,9 @@
 		endif;
 	endforeach;
 
+	// opportunity for plugins to add URLs to be pinged (perhaps stored in post meta, etc.)
+	$post_links = apply_filters('pre_ping_post_links', $post_links, $post_ID);
+
 	do_action_ref_array('pre_ping', array(&$post_links, &$pung));
 
 	foreach ( (array) $post_links as $pagelinkedto ) {