Don't forget Mad Magazine. Do they even have to get permission to draw half the stuff they draw in the background?
Check out this Mighty Mouse cover Too bad it's all fuzzy and small... but there are a lot of Terrytoons characters there (the ones they have the rights to) but you can see a lot of others that aren't. Like Cecil the Seasick Sea serpent. There are a LOT of others that aren't clear in that piacture, but I saw Underdog (from the bottom down) Rocky AND Bullwinkle, George of the Jungle... can't really remember who else. But you can tell they snuck away with it. Sometimes a drawing is just a drawing. Done right, no one will bother calling you out.
Now, if they were to run a story about one of these characters, they would no DOUBT be caught. You can sneak things into background... just be careful.