Creating a new thread based on Jem's comment in the "Githyanki and... Food" thread:
There are the moral questions -- should we disapprove? To what extent? Should diapproval motivate active opprobrium? Beginning to shade into the practical -- is it cause for just war? How should interspecies communities like Sigil cope? Should the shipment of bodies be permitted? What other questions arise?
While I don't like the PS novels in general, I think this was touched on in "Pages of Pain." Basically, at least in order to serve 'long pig' in an eating establishment, there had to be some kind of approved source for the meat (in the novel, I think it was something like 'found on a Blood War battlefield'), and the restaurant/establishment had to have a license for it.
My take on the 'long pig in Sigil' issue:
The Guvners maintain a list of which species are considered 'sentient' with regard to 'does it count as cannibalism?' This list is updated each year. Grey areas include a lot of the weirder species crosses (and on the Planes, there are a LOT of weird interspecies crosses), as well as some extremely rare and exotic species.
An eating establishment requires a license in order to serve 'long pig.' This costs jink, and a Guvner inspector will be around occasionally to make sure the eating establishment is in compliance. A proper source for the meat must be provided. (There is a list for this too, but there is a section where the person can write in a source not already listed.)
Some eating establishments, especially in the Hive, serve 'long pig' without obtaining a license to do so. There is also a problem with 'long pig' being served as pork or sausage.
As for the moral plane, it depends on which planar you ask. I imagine a typical nonevil approach might be "it's normally not allowed, but under extreme starvation conditions, it's ok." A good-aligned priest or paladin might be forbidden to eat 'long pig' even if he faces starvation. On the other hand, eating the dead might actually be considered a legitimate 'burial custom' in some societies.
Actually, the Guvners probably allow cannibalism for religious purposes under a different set of rules.
The basic idea of a centrally-maintained list forbidden to most establishments makes sense to me. It's probably the same list used to determine whether a killing is murder. Since the primary purpose of Sigil's laws, as I see them, is to make the city as attratcive a place as possible for as much trade and traffic as possible, that list is probably pretty broad-minded.
To be in compliance with legal requirements for the provision of manflesh, I suppose the Guvners would primarily care that the "meat" had been "butchered" in accordance with local laws, and outside of Sigil at that. However, some of these local laws could be tyrannies on the order of "I can kill and eat whoever I want," which I doubt the Guvners would recognize as legal systems. So you'd have the second layer of such imports only being permissible from entities that the Hall of Speakers or the City Courts recognize as legitimate governments, which would probably not overlap very far with cannibal cultures. You might get a lot of ongoing wars which are simply fig leaves for body-farming under "prisoner of war" clauses.
Obligate anthropophages (creatures who cannot substitute other meats for intelligent prey) pose another difficulty; it's probably reasonable to assume that they're expected to feed only at licensed establishments, and that these are generally shunned by other citizens and pushed as far underground as possible.