What are your thoughts on an interface to grant superadmin status (phpBB has an interface like that,
I believe)?

Founder status is the "true fail-safe", although that might be simplified eventually.
Superadmin status can be incredibly powerful, and it's normally possible to assign all of the
permissions assocciated with it to regular admins which makes it more of a "protection" flag for
admins along with allowing superadmins to edit all admins regardless of admin level.