I managed to find a video on Youtube of the Shubibinman 3 section I'm referring to (skip to 12:30). My Japanese is pretty shitty, but she brings up the hometown and pen name of each artist, and I'm pretty sure she says that some are 11 and 12 years old and something about the staff being moved by this before it moves onto something else. So I don't think this is the standard sort of omake stuff you usually find (there's plenty of that in the end credits and after this section), but if someone who's far better at Japanese than me wants to give it a listen and let me know if I'm totally misinterpreting what's going on there, that would be great.