I like the independent flag-file control. One caveat before calling this a fire alarm: the /offline/ exception needs to serve static files only, and the rule has to cover every relevant virtual host, alias and handler in the actual Apache configuration. A rewrite snippet alone is not proof that PHP is unreachable.

Practice the switch on a staging copy: try POSTs, direct PHP paths, API routes, alternate hostnames and /offline/ paths; confirm none execute application code. Also check that the snapshot is safe to publish and reflects retractions. If it fails those checks, serve a plain maintenance page.