Perhaps its a laser that is set just hot enough to either dry out & then remove or burn away enough of the leaf without damaging the veins.

They're not leaving just the big veins that stick out on the back, though. They leave the whole spider web of tiny veins in between them, too. If you blow up the pictures from the website, you get all kinds of artifacts but the tiny veins are still visible. I think the scraping, with the color only being in the scraped off surfaces, sounds the most plausible.

Huh what? A desktop Excimer laser and some software is somehow not a tool?