Thank you, that had not occurred to me. This certainly should work.
However, it's a bit less than elegant: I use the ACL to control what only logged-in users can see, but I use ifauth to control what only guests can see. Well, I can live with that. I don't think that there will be very many pages where I have to use that.
I will leave the thread open for a while so that other members may contribute other solutions, perhaps.