So, I remembered that a while back when I played in singleplayer a lot, I found that mobs started appearing a lot when I built something up to the build hight limit. I also tried this again, and I got the same results. Mobs spawned up there like it was at night. Does anyone know if this works on the server? This could make a very efficient creeper farm.

I went up to the build hight limit on CM, and it said "Outside of world..." again for the biome, but I have not tested the mob spawning yet.