I don't think it would be too hard to write your own code to do this actually. You can get a list of all the tab titles, and put that into a combobox fairly easily. If you sub class JTabbedPane you can use the getTabCount() and getTitleAt(int i) methods to find the number and titles of the tabs. Put these into a String array and use that to construct a JComboBox. Then customise the rendering to display the combobox or button too.
Of course, you would have to update the combobox when a tab was added or removed as well.