apache 2.4 – Serves static content, but with a 503 response code

We are currently leading our application cluster with a reverse proxy, and each rising block has an alternate host that only serves a static maintenance page. So, if all backends fail, return this page with a 200 OK, which is not the ideal return code for several reasons.

If my life were simple, I would simply set ErrorDocument 503 /index.html and continue with my life, but the problem is that we serve this site in English and French, and therefore we have 2 different error pages. The mod_rewrite configuration is:

  RewriteCond %{REQUEST_URI} /EN (NC)
  RewriteRule ^/en$ /index-en.html (NC,L)

  RewriteCond %{REQUEST_URI} /FR (NC)
  RewriteRule ^/fr$ /index-fr.html (NC,L)

  RewriteCond %{HTTP:Accept-Language} ^fr (NC)
  RewriteRule .* /index-fr.html (NC,L)

  RewriteCond %{HTTP:Accept-Language} ^en (NC)
  RewriteRule .* /index-en.html (NC,L)

  RewriteRule (.*) /index-en.html (L)

My next opportunity was to add R=503 to RewriteRules, but this causes Apache to return the default 503 ErrorDocument, and not our custom content.

To further complicate this is the fact that the "static" content on these pages is periodically regenerated from our CMS and cached to keep it updated, so I cannot simply leave a fast and dirty PHP script in front. this either.

Is there any way to achieve this purely through the Apache configuration?