We use something similar on my company website. In cases of a complete failure on the site we have something like:
RewriteCond %{REQUEST_URI} !/offline.html
RewriteRule .* /offline.html [R=302]
This, as I'm sure you can see, redirects anything that isn't offline.html to offline.html, apart from itself (preventing redirect infinite loop).
You could use something similar, only rewriting if if it isn't index.php
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