Seems like a great use case for a slack bot. Monitor for naked hellos and show a link to this page. That way it’s not personal.
It would need to be _polite_ though since most people who do this do seem to be generally well-meaning, if unwittingly annoying.
Please share if you do build it, I’ve been looking for a PM auto responder for vacations. They exist but many of them are bundled to other services with features I don’t care about and come at a cost that’s more than I want to pay for just the one feature of actual interest.