If I light up a large area of the Nether (so that the light level is >7), and give it sufficiently low ceilings to prevent Ghasts spawning, can zombie pigmen spawn?
Found this on the MinecraftWiki entry for Zombie Pigmen:
Spawn Any 2 block high space in any light level in the Nether. [...]
Minecraft Wiki says "Even with no light sources around, the player can see in the Nether, due to apparent "background light". However, this doesn't count as light for other game purposes—lighting up areas is still useful to prevent mob spawning."
I have lit up some parts of a Nether Fortress and not had anything spawn there yet, but I haven't tested for long enough to have any certainty.