Configure nginx to serve a 503 if a file exists
Solution 1:
From this page: https://calomel.org/nginx.html
## System Maintenance (Service Unavailable)
if (-f $document_root/system_maintenance.html ) {
error_page 503 /system_maintenance.html;
return 503;
}
The important part being the '-f', I believe - it tests to see if the file exists.