Ahh cool. I'll give that a try, printing out the EntityList mapping thing. Interesting idea.Mods that add mobs and use their own EntityList (such as my mod) should use EntityRegistry.registerModEntity, it adds a mapping to the vanilla EntityList class which has several static maps, such as EntityList.stringToClassMapping, if you loop through one of those you'll get what you need (it normally adds them as 'ModID.MobName' for example 'DemonMobs.Belph'). Some mods might not use the forge registry in which case they can still manually add their mobs to the list or they just use the vanilla list, in this case it'll just be 'MobName'.
I think changing the spawners is just a matter of changing the mob name from there, I'm not entirely certain though.