Yes, document-specific RibbonX addons are possible in the Visio 2010 TR. We haven't posted the documentation yet for the requisite API's, but that is forthcoming. In a nutshell:
1) addon implements IRibbonExtensibility interface (as explained on MSDN)
2) addon calls Visio.Application.RegisterRibbonX() to register the interface. RegisterRibbonX() takes two arguments -- a Visio document and a visRXMode enum. If NULL is passed for the document, the addon's RibbonX UI applies to all documents, otherwise it applies to the specified document. The visRXMode enum allows the addon to be further narrowed to a specific window type or mode (e.g. Drawing, Stencil, Print Preview, etc.)
3) upon shutdown of the addon, addon calls Visio.Application.UnregisterRibbonX() to detach the addon UI from Visio. Note this must be done early enough in the shutdown sequence so that the application and document objects are still available.
FWIW, all of the Microsoft-authored Visio addon UI is implemented in this manner. Org Charts, Database modeling, etc are a couple examples.
If your RibbonX does not require addon code and simply shuffles the UI around, you can attach the RibbonX XML to the document via the document.customui property and then save the file. Upon load of the file, the Ribbon XML stored in this property will be realized in the UI. If you're exploring the API changes via IntelliSense/Object browser/Reflection/etc, you will also notice the the existence of a 'usercustomui' property hanging off the document. UserCustomUI is where the user's customizations are stored. Users customize the ribbon via File | Visio Options. And yes, all methods of Ribbon customization (RegisterRibbonX(), Document.CustomUI, Document.UserCustomUI) can be simultaneously employed to reconfigure the UI.
If you choose to use the RegisterRibbonX() API to register your addon ui, then you have the luxury of not needing to create a new COM Addin (like the rest of the Office Apps) and instead just need to implement the IRibbonExtensibility interface on top of your existing VSL. Creating a COM Addin just to customize the Ribbon will result in an addon that is inherently not document-specific. Sticking with a good old VSL is the only way to get a document-specific addon.
Hopefully this is enough to unblock you.
Barry