For philosophical reasons, the author of s6 would prefer to leave a configuration language out of s6, and left as an exercise for the user. Here is one such implementation based on TOML, along with some extra goodies like automatic log pipeline creation.
[2024-03-04] NB: seems like it was forthcoming at one point, c.f. https://skarnet.com/projects/service-manager.html but the author hasn't committed to the effort since 2022.