Is it possible to add a custom button to the Document Locations associated view?

I've tried using the ribbon workbench, adding a new button in the SubGrid panel of the SharePoint Document Location entity, no joy.
I've also tried exporting the default solution and scanning the customizations.xml for "Mscrm.DocumentsTab" which according to MSDN is the ribbon, but it's not in there.
Has anybody successfully achieved this?
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