>In fact, despite many minutes of bona fide web searching, I was unable to locate the C++ signature for IRibbonExtensibility.

Probably because the COM "intended" way is to generate them from type library. Type library for these interfaces is embedded in Office MSO.DLL. You can use oleview.exe from Windows SDK to convert them to IDL syntax. This yields such signature:

    HRESULT GetCustomUI(
                            [in] BSTR RibbonID, 
                            [out, retval] BSTR* RibbonXml);

    DECLSPEC_XFGVIRT(IRibbonExtensibility, GetCustomUI)
            /* [helpcontext][id] */ HRESULT ( STDMETHODCALLTYPE *GetCustomUI )( 
                IRibbonExtensibility * This,
                /* [in] */ BSTR RibbonID,
                /* [retval][out] */ BSTR *RibbonXml);

I actually stumbled upon that toward the end of the exercise, but couldn't figure out where `oleview.exe` landed after I installed the SDK. I only spent a couple of minutes, and at that point I was only looking to confirm what I already knew, but this will be the approach if/when we extend it.