Currently adding a custom set of attributes to all LivingEntityBase for my Dynamic Stealth mod (ie. adding a custom attribute to vanilla / unknown mod entities). Had some trouble before I found a usable event for doing so, but I believe it's working now, so figured I'd chime in:
The event for doing such a thing (by which I mean, the ONLY event that works, when the entity is reloaded from disk, afaik) is
EntityEvent.EntityConstructing
If there is another event with the correct timing, I do not know of it. the join world event is too late to add attributes to the attribute map without warning spam in the server log.
Hopefully someone finds this useful