On Wikimedia wikis, where MediaWiki-extensions-CentralAuth is used; the bot accounts used by MassMessage do not even appear as "unnatached" on https://meta.wikimedia.org/wiki/Special:CentralAuth/MediaWiki_message_delivery. This seems to be a bug of some sort.
New MassMessage system users should be automatically attached to CentralAuth upon new wiki creations.
This also allows local projects to get GlobalUserPage descriptions of those system accounts, reducing confusion.
Thank you.