I was watching a live stream of someone creating a railway through the Nether when it was suggested to him to prevent zombie pigmen from spawning on his rails by placing button on the space above the rails (either attached to the ceiling or the wall).
Having never seen buttons being used for that purpose I started researching it a bit. Buttons do count as transparent blocks, but mobs can spawn on pressure plates, so perhaps redstone components are an exception. The relevant question on this topic on Arqade did not list them specifically in the answers (see Is there a way to keep Zombie Pigmen off of minecart tracks in the Nether?).