To add to the anecdotes, I have been sleeping on a flat rice straw tatami matt (without anything else) for 2 months. It's pretty hard and I have bruises on my ribs, but I have way less back and neck pain than before. I usually fall asleep in position similar to Figures 4 and 5 in the paper.

The reason I started doing this is that after moving to a new house I bought an expensive memory foam mattress which made my back hurt every morning.