The only trigger that can be used to modify activity is activity_onDisplayStreamActivity as follows.
Code:
$_PLUGINS->trigger( 'activity_onDisplayStreamActivity', array( &$row, &$title, &$date, &$message, &$insert, &$footer, &$menu, $stream, $output ) ) );
Those are all the variables that are available to you. If none are sufficient for your needs then you'll need to modify the template files.