For those who are interested and may not know; Bill Watterson also ghost-drew a few strips for the comic Pearls Before Swine (part of an arc beginning on the 2nd of June 2014[0] and continuing till the 8th). There are a couple of neat references to who it is behind the better art (anyone who's read Watterson's complaints on shrinking panel space in print media will be familiar with the guest character's comments) and I recall an old blog post by the artist of Pearls describing what it was like to work with Bill (terrifying; the thought of the postal worker just chucking these, the only comics anyone had gotten out of Watterson in years, onto his porch in the rain was horrible), but I can't find it.

[0]: https://www.gocomics.com/pearlsbeforeswine/2014/06/02

EDIT: Nevermind, found it! https://stephanpastis.wordpress.com/2014/06/07/ever-wished-t...