Thought I have seen the whole leaf in poop, I have also see slightly broken down. I could see some things getting digested. I do not believe the plant eating is what we think. I have been watching my female and she has been eating less plants, even though on a diet with decreased food. To me this suggests plant eating is not a food source. Now weather it is digestive, for moisture, or just something they do. I do not know.
Point is just by chewing I am sure some is digested, particularly Yemen Chameleon
Some plant eating is a result of the tongue getting stuck to the leaf when eating or drinking. Some is speculated to be used to clean the gut/aid in digestion to some degree.
My female veiled enjoys eating plants. She enjoys kale quite a bit - but she will strip a plant bare tasting it (nips / rips leaves and/or goes for the stems). No clue why, but she eats a lot of foliage.