While not recommended because there can be two different  commandbar popups with the same name, you could use DTE.CommandBars[commandBarPopupName] to retrieve the commandbar of a CommandbarPopup from the DTE.CommandBars collection. However, the new WPF-based commandbars of Visual Studio 2010 introduce a performance problem when doing it so if the commandbar popup has not been displayed in the Visual Studio user interface.

This new article exposes the problem and provides proper ways to retrieve a commandbar popup:

PRB: Performance problem of DTE.CommandBars[name] for CommandbarPopups in Visual Studio 2010
http://www.mztools.com/articles/2011/MZ2011005.aspx