Yes, my problem was that I had set my custom print size to 610mm wide, which I thought was the metric equivalent of 24". Turns out it will print fine when set to 609mm, but not 610mm.
Basically (it seems to me) that the driver then thinks that since the image is larger than the paper size, it should tile my image across multiple sheets of paper, and uses the set (greyed out) paper size in the "Paper Handling" tab as the size sheets it will tile the image across—in my case, US Letter.
As soon as I set my custom image size to 609mm wide, it prints fine :-)
Thanks again Roy!