I did exactly as others have suggested: created a fake email account (this was pre-Gmail; I think I used "sithmail") called [author name]reviewer, and corresponded that way.
Actually, I did this not to ask about the paper, but to strongly recommend to the author that he put his article on the arXiv before signing anything (and pointed out that the editor himself frequently posted to the arXiv). I didn't see any point in putting the editor into a difficult position, about whether he was allowed to suggest to the author that the author arXiv the article.