Creating redirects for files (images) on Pantheon
Thanks to Sarah at Pantheon for walking me through this.

File redirects aren't really possible using the same logic used to do PHP redirects for URLs.  Requests to files are managed by the server and don't run through settings.php (Drupal) or wp-config.php (WP).  It is not possible to intercept those like a URL served by the application.

If you were running your own nginx server you could manage this in a .conf file, but on Pantheon the nginx.conf isn't editable on a per-site basis since it's set platform-wide (https://pantheon.io/docs/platform-considerations/#nginxconf)

A workaround could be to utilize a third-party CDN like Cloudflare to create nginx-like redirects before you hit the server.