Perhaps only have a fakeannounce permission*, and make vanishing persist between sessions? That way you could vanish and give a "left the game" notification, then leave the game for real - and on join, you would stay vanished and the presence of the fakeannounce option would silence the join message.

*Or indeed a fakeannounce toggle command, as it's annoying having to ask the server owner to modify my permissions whenever I want a little extra covertness.

Changing to persistence sounds like fun. Also irritating to write. But fun!

For now, burying this.

had similar options in https://github.com/MilkBowl/Administrate before I stopped developping, feel free to check it. may not be what you're looking to do though.

2.3 splits silentjoin. Finally