@gwensnyder in a post click the little globe at the bottom and set to "mentioned people only". It's not quite the same as a DM, but close - if you @ mention someone (besides the intended recipient) they still get access to it
@joshuagrochow@gwensnyder Replies to such messages will have that same visibility - mentioned folks only - by default, so you can keep a running DM thread without fiddling with that setting when you reply.