Is there a mechanism or hook in place to update an external metadata store when an admin makes changes to an identity (e.g. creates, deletes, updates traits, etc). I know there is a hook available for when a user updates their own traits, but there doesn't seem to be an equivalent for Admins?

No, I would expect that the part doing the admin API call can also do the update in the external store as well.

So I would need to build a layer in front of the admin APIs in order to achieve this?

Aren't you building that any way because of the required authentication for the admin API?