I am trying to add button(s) to the Visual Studio / TFS2010 Work Item query results menu bar and also to the menu bar shown for individual Work Items (see image below) - is that actually customizable & doable and if so, how?

I would take a look at this post:
TFS 2010 - Adding a menu item to the Completed Builds context menu in the Build Explorer
It discusses the steps you need to take to add menu items to context menus.
To answer my own question (as the one by NGM was helpful but it did not directly answer it.) - no it is not possible as the Work Item & Query Windows inside Visual Studio, particularly their toolbars, are not callable as commands in a VS (SDK) sense.
Basically they are merely WinForms controls (well, were for VS2010, I think this has changed to WPF in VS2013 by now) placed inside a WinForms form hosted inside a VS ToolWindow. The toolbars I outlined above cannot be customised via the VS SDK.
This may have changed in VS2012 or VS2013, but for VS2010 there was/is no way to add custom items into the two toolbars I asked for back then.
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