I created a ticking area explicitly "non-spawnable". I removed any land including bedrock for nearly the chunk limit I can see all around and built it on a platform of bedrock only. The water in the background is just from pools from the time I removed all blocks from that area.

As you can see from this image, I have only bedrock, command blocks, redstone on top of the command blocks, torches, and a light detector. Somehow a skeleton spawned.

Can mobs still spawn on bedrock or command blocks? The texture pack is BDCraft, but of course this should have no impact.