nginx caching: return cached page (when specific Cache-Control header given) only if the upstream server is not reachable

I'm trying to set up caching for an upstream server (which I do not manage). Most files can be cached (and don't have Cache-Control set), those work fine.

However, some locations on the server are directory listings (and have Cache-Control: no-store). I'd like to cache those only if the server is not reachable. Unfortunately, I either end up in one of the following:

• In a situation where those listings are not cached (no file in the cache, header always shows a cache miss). If the server is not reachable, the directory listings are (obviously) not returned
• In a situation where those listings are cached, but they never update afterwards (at least not as long as the cache is valid). Since I'd like to cache all of the other entries for a long time, the directory listings become outdated quickly.

I tried to modify the headers to stale-if-error, but that didn't seem to help either.

map $http_cache_control $http_updated_cache_control {
  no-store stale-if-error;
}

server {
...
  location /somewhere {
      sendfile on;
      sendfile_max_chunk 10m;
      tcp_nopush on;
      proxy_cache keyzone;

      # allow using stale requests in case of errors or when updating a file
      proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
      proxy_cache_revalidate on;
      proxy_cache_background_update on;

      # add header to indicate if caching works
      add_header X-Cache-Status $upstream_cache_status;

      proxy_cache_lock on;
      proxy_read_timeout  900;
      proxy_pass_header   Server;
      proxy_ignore_headers Set-Cookie;

      # allow caching of non-cacheable entries only when the server is erroring
      proxy_hide_header Cache-Control;
      add_header Cache-Control $http_updated_cache_control;

      # don't ignore the cache control header: some items (like directory listings) are marked as "don't cache")
      #proxy_ignore_headers Cache-Control;
  }
}

How can I cache entries with Cache-Control: no-store, but only use the cached entries if the upstream server is down?