We all know that any sod addlecoved (or simply stupid) enough to be discussing the Lady's matters in Sigil will get himself mazed, flayed, or worse (see Rowan Darkwood as an example of the latter). However Sigil is just one spot, albeit a very important hublike one, on the Great Ring. Belief and knowledge hold just as much water on a farmstead in Bytopia as they do in the Hall of Speakers. So how does the allpowerful Lady stop berks from leaving her little ring and talking trash about her outside of her influence? I have a few ideas lurking in the old bonebox, and I'd like to hear both opinions on those (listed below) and especially new ideas that you come up with.
1) She doesn't. You can blabber on about her all you want, but she just seems to be immune to the changing beliefs of those outside her cage.
2) She has agents do her dirty work. Much like the dabus are her janitors, there is an enigmatic and poorly known race of assassins that targets only those who flap their bonebox too much on the wrong topic, and they are never seen in Sigil (not that they're seen too often anywhere else, but their work is).
3) Her influence is greater than people think, particularly when it comes to portals. Even if she doesn't flay you outside of Sigil, she can at least make you appear there after you walk through that next doorway.
4) Everyone is too paranoid to talk or listen to fluff about her Serenity, and only the vilest most powerful fiends dare plot against her, knowing that she cannot stop them outside of her domain.
5) The spiders... AAAAAHHHH! The spiders! *Pop..fizzle..hiss....*
It's an interesting question. Can you worship the Lady while safely outside of Sigil? Could there be a cult that worships the Lady but never steps foot in the city? Is Sigil's design somehow insulated from belief?
One thing we know--belief does not tend to extend very far from the believer. Signers excluded, it's very hard to do something to a plane by gathering a bunch of believers on another plane and thinking about it really hard. You need to be there, in the thick of it, to make things happen. Otherwise, you're probably going to need a hell of a lot of people.
If you're going to modify the Lady of Pain from outside of Sigil with belief, you're going to need a hell of a lot of believers--because they're going to have to overcome the combined beliefs of the dabus and all those inside Sigil itself (who obviously believe the Lady of Pain isn't someone to worship). And since they're actually there, it's quite feasible that you won't be able to pull it off.
My general feeling on the matter is that she doesn't need to bother with people outside of Sigil, because no one would ever be able to gather enough believers together to have any effect on her. They'd have to overcome the dabus (which may explain why she's so fierce about keeping them loyal).