Is there a way to message another HN user, to be able to communicate without it being public but also without giving the world your email address?
No there's no "DMs" on HN as far as I know. I was tempted to add my mobile number, but being the public internet and all probably not the wisest choice.
(A day later, the Washington Post runs an article casting shade on direct messaging since it can be used to say bad things.)