<p>I've seen with Microsoft COM and XPCOM, at least from what I've read and gathered so far, that the implementations of interfaces in a component have to essentially be in the single class that derives all the virtual interfaces. Is this correct? What am I missing?</p>

<p>Is there a way to have multiple objects (possibly in separate DLL's) each provide their functionality and still be able to freely transition between them using QueryIterface?</p>

<p>What I'm looking for is to have a component with some functionality, but still allow external client code to create new extensions of the component with (possibly) new interfaces. Ideally this should happen without divulging the current source of the component and its implementation.</p>

This should be possible, although probably not supported by the standard high-level wrappers. Most of the wrappers (ATL, MFC, etc.) only support mapping a COM object to a single class. However, QueryInterface is allowed to return a different pointer and calls COM object code, so the first COM object could load a different DLL, instantiate a different object, and return a pointer to it's interface (vtable).
It's all possible as far as I know, you'll just likely be writing a lot of the low-level glue code yourself.