put website mirror links on reverse proxy error pages
The idea here is that if the origin server is unreachable for whatever reason, we'll display the normal error message (eg. Bad Gateway), but also present a list of mirror sites and that error page.
Of course we'd first have to establish these mirror sites. For this use-case it doesn't matter whether the sites are a full mirror of f-droid.org or just a small landing page for providing the most basic features.