As part of the Map Maker's Toolbox project I've created a way to have mobs drop 100% custom loot. 
It supports multiple kinds of the same mob (each one dropping a different kind of loot), chances, different stack sizes of the same item, etc. 

To use it, simply open the schematic in MCEdit and place it in your spawn chunks. Remove and replace the redstone blocks to start the fill clocks that run this device.
So, if you're creating maps and would like to have mobs drop anything you like them to drop, you can download it here. 
(In the video I refer to my Github repository which is an alternative place to download the schematics from.) 

A few important things: 

• DOES need to be inside the spawn chunks to work. 
  
• Can be placed multiple times in one world and work as intended if you set it up to use different CustomNames
  
• Gamerules needed to make this work as intended are: doMobLoot, doTileDrops, commandBlockOutput all set to false.
  

Feel free to use this anywhere; there is no need to credit me unless you're doing something that focuses around the contraption itself such as creating a video about it.