I used to be able to do this by overriding the registerItem for the item's class to add custom ItemStacks to the passed list- but the passed list is no longer present. How do you go about doing this now?

In you Item you override getSubItems method.

 

There you add new ItemStack to a list of subItems that will be then displayed in tab this item is assigned to.

ItemStack can be whatever you want - you can add ench, NBT, metadata

Thanks. So it turns out I was trying to override the wrong method