Office Custom UI Editor for Microsoft Office tool has been made available and is attached to this article. The highlights are:

1.  Office custom UI editor is a tool to edit the Office custom UI XML part in the Open XML file format.

2.  This version adds support for dual parts where one is Office 2007 custom UI and the other is Office 2010 custom UI.

3.  Contains both Office 2007 and Office 2010 custom UI schemas.

This is a refresh of the tool featured in the below article:

http://msdn.microsoft.com/en-us/library/cc508991(office.11).aspx#UsingtheCustomUIEditor2_AddingTemplatestotheCustomUIEditor