EntityLiving#onInitialSpawn Event

I'm sorry if this has already been suggested. I wanted an event that happens at the end of EntityLiving#onInitialSpawn, so that I could add armor to the Entity. If the entity is spawned with the /summon command and the user specifies that they want certain settings for the Entity, I don't want to interfere in that scenario, and neither does Minecraft. This is why Minecraft checks if the argument length is greater than or equal to five to stop this from happening. I essentially want a nice way of adding to the pool of random Entity equipment some get when they spawn, without interfering with someone's wishes of an Entity being summoned with specific equipment. If not this, I'd like maybe some registry to add armor to the pool of randomly assigned equipment of an Entity, like how Fishing treasure, fish, and trash are added.