Aha! So, if there’s never been a woman (I also searched and didn’t find any evidence that there had been one), then we have no conclusive evidence as to what the protocol would be, were a woman to hold the title of editor-in-chief.
But it does seem quite obvious that if you’re writing a letter to the editor, and it’s a man, you would address it to “sir”. Mystery solved! Drinks all around!
(oh wait, this isn’t a bar, it’s the internet, and i’m supposed to be finishing a lab right now)