Thanks for testing this guys.
Italophile, I played with your template and didn't see the tab so I customized the code a little bit to get it showing but I still didn't see the behaviour that I was hoping for. At one stage the file opened with double-click working but then I couldn't reproduce that again. Does this version of the customui code work on your machine?
Code:
<customUI
xmlns="http://schemas.microsoft.com/office/2009/07/customui"
xmlns:mso="http://schemas.microsoft.com/office/2009/07/customui" >
<ribbon>
<tabs>
<tab id="newTab" label="Hilite Test" keytip="H" insertBeforeMso="TabHome">
<group id="newGroup" label="Test" >
<mso:gallery idQ="mso:TextHighlightColorPicker" showInRibbon="false" visible="true" size="large" />
</group >
</tab >
</tabs>
</ribbon>
</customUI>