or is it okay for comms/journals to do their own thing and then say here "hey we're doing something"? Or the latter isn't expected, things should just be tagged?

Exactly this :) you can choose to just tag, or tag and promote, that's entirely up to you. Very happy to hear you've got something planned!

(People are welcome to run events here if they wish, so long as it's a broader scope - nothing fandom-specific. )