We gave up.
After about a week of hoping that Muriel would use the cat box in the garage (and cleaning up afterwards) we got a small litter box that we can wedge into our small bathroom. I suppose that it's useful to have the box so close to the toilet. What's weird is that she's figured out that the food is in the garage -- I can't figure out what was keeping her from using the cat box there. And she's certainly figured out how to rattle the cat door to achieve maximum sleep disrupting effect. Oh well. Muriel is truly an irritating creature in the shape of a cat.
In more pleasant news, I continue to marvel at how deeply the winter sun penetrates into the house now that we've removed that aluminum awning. I can only conclude that the previous home owners were trolls, or only lived here during the months of July and August.