Having a list of open sheets directly in the ribbon doesn't seem to work with this type of UI. You can code an alternative quite easily though. Add a button to your ribbon to access the list of open sheets. When clicked, loop through all your open sheets and dynamically generate a pop menu. Then just display the pop menu where your ribbon button is. I'm not terribly familiar with this Ribbon but maybe you can dynamically add child items to the ribbon button itself?

Hi Stephan;

  Only the menu object supports this automatic feature. For the Ribbon Bar, you would have to custom program this behaviour.

   Conversely, if you leave the menu in place and remove it's toolbar properties...adding user object(s) to mimic the Ribbon Bar would allow the menu to function as before. Then the UO would have the Look & Feel of the RB and would be fully inheritable.  ;-)

Regards... Chris