Client-Side Equivalent of drupal_set_message with javascript
I don't see JS API for dispaying messages even in D7. Maybe because all you need is DIV with classes like messages,status warning or error. You can see how drupal works with it in file /misc/tabledrag.js.
When you drag rows in tables drupal shows you a warning message with this code:

$(Drupal.theme('tableDragChangedWarning')).insertBefore(self.table).hide().fadeIn('slow');