Searching for all the silent ClassNotFoundExceptions generated, as discussed offline with Karl Pauls
It seems like the sling models service injector is using its own bundle context for the lookup. That makes this case reasonable problematic. Typically, an extender uses the bundle context of the bundle it extends - which would in this case make the lookup likely not run into any CNFE for the assignable check. If anything, you could try there and see if you could make it use the bundle context of the extended bundle.