(Still, I'd like to know whether there is such a thing like a global directory for smartlists...)
No, there is not.
Many people think they need separate libraries. In almost all cases, it's a bad idea, predicated by their lack of understanding of views.
You need to use views. If you want to pretend you have separate libraries, then instead of corrupting the Genre field, you could create a field called "FakeLibrary" and assign every file in your real library a value in the FakeLibrary field according to which fake library you want them to be in.
Then simply modify all your views to restrict to a particular FakeLibrary value.
Once you do this, one possible FakeLibrary value might be "Global" and you could configure anything tagged thusly to appear in all your views.
Simulating separate libraries in this way is unnecessary, but it makes some people feel better.
You cannot tag playlists/smartlists, but you can place them in playlist groups. Playlists/Smartlists are tied to a library, but they are "global" within that library.
You are only having this problem because you made the mistake of using separate libraries for different types of music. If you used views to segregate your music, you would have noticed your playlists were already "global" and you wouldn't be here now asking this question.