I've been playing around with creating a command block module to add harder mobs to the game. I've changed the loot tables of these mobs using entitydata.
I have attached an image of the command blocks.
/entitydata @e[tag=RHM_Zom]

{CustomName:"Zombie King",CustomNameVisible:1,DeathLootTable:"custom:rhm/zombie"}

This command is run in a chain command block, it shows as successfully run but the mob doesn't drop anything when killed. It does sometimes work though, which is odd.

I ran the command again in a separate command block and it worked properly, making the mob drop the different loot.

I can provide a more in depth explanation of the command block module if its needed.