ACL level for Expert mode but not superuser

Based on commit c97177e (from issue #2094), regular users (who may have limited page permissions) can no longer edit the YAML frontmatter within the page editor. But there doesn't seem to be any way to give users complete access to pages (so editing the frontmatter wouldn't run into security issuess) but prevent them access to the rest of the admin, e.g. plugins. It would be particularly useful to have an appropriate access level below superuser but above regular user that enables expert mode.