This last treatment is less an example of a practically useful technique than it is a creative reinterpretation of something very straightforward. I was just playing with an eight-color Potrace tracing (Path ▸ Trace Bitmap, 18.8.2 The Trace Bitmap Dialog) of this area of the original photo when I noticed that it acquired an interesting map-like look when I discarded the fill colors and assigned a dashed stroke to the resulting shapes:
All that was left to do is create a yellowish, slightly rough (Filters ▸ ABCs ▸ Roughen), semitransparent (so that the background drawing shows through a bit) background rectangle to imitate the surface of an old map, and add some cryptic text labels to complete the picture.
As a final touch, I added dotted frames to all five squares containing various treatments. The final image is on the cover of this book.