Bumping this one, I am still trying to find it about SSO and Discourse and Wiki.js.
Edit: I found this: Setup DiscourseConnect - Official Single-Sign-On for Discourse (sso) - admins - Discourse Meta
Which seems that it should be able to work with some sort of SSO, but I unfortunately lack the skills to integrate this with ssowat :-(.
Edit2: Solution posted here, thanks: Discourse: autologin with SSO