I actually really love seeing other manga artists do characters from other series. I wish they'd make some of them into posters or artbooks, because I think they're the most fun to look at. Shows what other creators' favorite characters are too.
In the past, some creators have done their own Doujinshi with other characters. I thought Sonoda said he did that many years ago. Would be fun to ask for a sketch from any particular artist of the character he most likes from someone else's creations. I wonder if they would do it and just not sign their name?
They might not be more common because of copyright red-tape? Or maybe no one thinks there's a good market for em? I know that sometimes Arina Tanemura posts doodles she does of other people's characters to her twitter (her Sailor Moon ones were adorable!), so maybe some artists just save em for that kind of thing or doujinshi/sketchbooks they might sell at appearances/conventions?
I have that SAME first book, and ALSO did not know about the second one... the Yoko is really cute. I should also go scan a splash page from one of my Afternoons. There is a short they did that is apparently a collaboration between all the people that were in Afternoon at the time. It's especially notable (to me) because it has a splash page with Skuld as drawn by all the artists (and maybe even some that weren't in the magazine at the time).