brainCloud doesn't directly support loot boxes currently... Though of course the Item Catalog and User Items services give you the building blocks to construct your own system in cloud code.
To implement such a system, you would:
use Global Entities or Unowned Custom Entities to represent the collection of loot box templates
construct your JSON with the rules (percentages, etc.) for constructing the loot box
then write a cloud code script that when run - performs the work to award the user the relevant items
Note - there are future plans to add such a feature to brainCloud - but it isn't scheduled yet. I'll add your +1 !
Paul.