It would require a fair amount of hacking, but Mage: The Ascension has a lot of tonal fit with Planescape (factions jockeying for their philosophy to take control of reality)
It's a tonal match for Paranoia as well, for similar reasons. Everyone's in one or another of a collection of wacky cults, it takes place in a weird dystopia, the bizarre entity that leads the dystopia has forbidden people from worshipping it as a deity