i guess, you are using 1.72?
then it should be the correct place.
$comment_text = (isset($HTTP_POST_VARS['comment_text']) && $error) ? format_text(trim(stripslashes($HTTP_POST_VARS['comment_text'])), 2) : "";
$site_template->register_vars(array(
"bbcode" => $bbcode,
this is what you should search for then