Globals.Ribbons is a VSTO designer feature, if you use RibbonXML then you don't have this feature.
What the designer actually does under the covers is it will create ribbon xml for Office, then when office makes a callback, VSTO will raise the appropriate event handler for that context (document). Because you are using RibbonXML you are bypassing the VSTO Ribbon designer support entirely (I prefer it this way, it is faster and you have more control).
With ribbon XML you will have to register a onLoad callback for your label, Office will then pass you a IRibbonControl, which will be the label, and you have limited stuff you can do. If you wanted to change the text then off the top of my head you would have to register a getText callback, then invalidate that ribbon control, which will cause the getText callback to be reevaluated.
Having more info about what you actually want to achieve would be handy =) I have a feeling my VSTO contrib project will also make your life much easier, as it gives you many of the nice Ribbon Designer features when using ribbon xml. But let me know what it is you want to do, and I can give you more info about that.
Cheers,
Jake
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