I can't stand it any longer. I'm going to toilet train the cats, using the 9-step program outlined in this book, Kick Litter, by Perre DiCarlo.
The training method is so simple that it is explained in two pages. The rest of the book consists of photos of the author's cats and cutesy captions of what the cats "think" about the method. The book's cover jacket is an instructional poster you can remove and unfold, and contains everything you need to know to try this method.
I'll give it a try. If I'm successful, I'll shoot a video of my cats in action.