I don't know if this is the sort of solution you are looking for, but I treat all shadow-creatures as outsiders. Not mechanically, they keep their creature type and such, but from the perspective of the story. They can be summoned with incantations to perform specific tasks or for a specific duration, but they don't permanently inhabit our world.
This ties in with the illuminatus/kabala backdrop of my campaign premise. A secret society has been trying to find the incantation to summon Ashmodai from Sitra Akhara to take control of the world -- barring that, they have been trying to take control with the aid of lesser demons and "outsiders".
Many unsolved murders, plague outbreaks, catastrophic conflagrations, historically strange occurances, superstitions and even traditional religious rituals and practices are attributed to either the outsiders who have been summoned over the centuries, or the attempts by the good-guys in the know to counter the secret society's minions.
At the lower levels, before the PCs know what is really going on they will think that they just have to kill the monsters. After they know the deal, they will realize that they just have to kill them before they perform their appointed task. It all works out the same though -- it is a good excuse to find and kill monsters.
The monsters themselves aren't integrated into modern, daily life (no Gnoll Pimps) though, so it probably isn't what you are looking for.
Cheers