While I didn’t yet find the time to test it with the Dokuwiki plugin, I hacked together a SimpleSAMLphp IDP auth plugin that authenticates against the Nextcloud12 DB. More or less untested and not recommended by the Nextcloud developers, but maybe of some use for someone other than me:
Since the attribute names are all specified in the SQL query, it is quite flexible and can even pass on group membership in its current form.