I was playing with the MCP code for server 1.12.2 and added an advancement trigger with a custom namespace (I just copied BredAnimalsTrigger and renamed it/added the trigger method call to spawnBaby method) and, when testing, it correctly loaded and granted the advancement!

I though this would be a neat feature to expose to plugins.  Kind of like end-user-exposed events.  That way a plugin can provide documentation on its available criteria triggers and their format, then server owners could use these to make advancements with custom plugin triggers.

I could try to come up with a way this would look in the Bukkit API if needed, but I've never contributed before so I'm not sure how it works.