If I create a class implementing LootTable and then set it via Lootable#setLootTable it will not work (the entity will drop nothing). The implemented methods are just not called at all. 

However calling setLootTable with a enum provided by org.bukkit.loot.LootTables works perfectly fine.

If the classes must be registered I cannot find anything and it should get noted (maybe) within the setLootTable method. If it is not possible at all it should definitely get noted there.