Mutliple rewrites/redirects in settings.php on a Pantheon hosted site
If you have a reverse proxy service/server sitting in front of your Pantheon instance, it seems like that's the place to place your redirect rules. Rewriting/redirection is a common feature of reverse proxies.

For example, CloudFlare supports redirect patterns under its Page Rules manager. Pantheon has a few doc pages that bring up using CloudFlare so it looks to be a common configure they see used with their service.

If you have a Varnish instance in front your Pantheon instance, you can configure rewrite/redirects in VCL.