How can the foobar components cooperate?
I mean some kind of more sophisticated way than menu_manager->run_command( "...." ) like calling methods directly.

It's basically the same thing as using services from SDK:
Get an instance of a service with known interface, call methods on that instance.

... and without having its source code?

If it is legitimate, official interface you'll be able to get full source code and/or at least SDK (headers, documentation, library). Otherwise you shouldn't use it at all - if developer decides not to give access to component it's for some reason...