What exactly is the social status of undead in Sigil? Obviously, the Dustmen work with them at the Mortuary, but can undead otherwise freely roam the city without harrassment? (Assuming the undead in question isn't going about eating people or draining their blood, of course...) Can vampires, lichs, etc... openly participate in public life in the same way that other monsterous races like Baatezu and Tan'arri can? I would assume that they can, since Sigil tolerates just about everyone (and everyTHING) else and the Dustmen would probably demand equal rights for their undead allies. Are there Taverns and Inns that cater to the - ahem - SPECIAL dietary needs of ghouls and vampires? Any specific mentions of undead NPCs in Planescape material?
Undead in Sigil
Well, there are the drowned nations in Torment, hiding away under the city because of what they are. I had an adventure in one game where the pcs encountered the drowned nations and had to agree to help them strive for respectability in the city in order to close a deal - the bargain involved helping them petition to start a guild and promote their death metal band. Unfortunately the zombie band members went nuts during the production and gave in to their brain fever, leaping into the crowd and gnawing on their audience ... the whole deal went south after that.
In general, though, I see no reason why undead who can control their baser urges wouldn't be perfectly welcome in the city - so long as they didn't break any of the laws every berk should be aware of.
Cool, that's what I was hoping... Um, what about feeding issues? I assume a vampire could find somebody willing to donate blood and/or life levels, but ghouls eat flesh. Then again, there might be folks barmy enough to cut chunks off of themselves to feed a ghoul, or maybe even let the ghoul bite pieces right off ("Cleric! Cleric! A little healing, here!"). Maybe a ghoul could BUY the bodies of deaders from the Collectors or Dustmen...
If an undead is intelligent enough to know that one target is safer than another, it will know that if it wants to move around Sigil freely, it can't get caught feeding on the unwilling. Vampire cults that regularly "donate" in hopes of eventually being turned probably exist, and ghouls of a sort that, say, merely consume corpses, could work the Hive for meals. Those that require living flesh, or perhaps symbolically level drains as the consumption of positive energy, must either restrain their appetites or get their meals outside of Sigil's jurisdiction.
Unintelligent undead that instinctively attack any living thing are, of course, not welcome any more than a wild animal would be. Those that are under the control of a spellcaster would presumably be his property and he would be responsible for any attacks they cause. Those with maledictory auras like stench, gaze attacks, etc. will either control them (if they can) or face the consequences -- retaliatory assault for death or injury effects, and being confined to the Hive or Undersigil for things like causing nausea.
Sigil is a place where fiends can freely roam the city without harassment. Even the vilest undead is typically not nearly as evil as a fiend. So no, I don't see a problem with an undead creature in Sigil doing anything any other citizen can do.
Skall is an important undead NPC, but he keeps his nature a secret. Apart from him (and Tenebrous, who can't enter Sigil because of his semi-divine nature), no examples spring immediately to mind.