We don’t currently offer the ability to configure arbitrary redirects, though it is functionality we are considering.

You could run nginx as a service within your application using a Dockerfile. We have an example nginx Dockerfile and configuration available in our sample repos at https://github.com/digitalocean/sample-nginx. To avoid unnecessary load, I recommend keeping your existing routes for the huge application, and add new routes to nginx just for the parts you want to proxy.

There is another similar discussion related to routes & redirects that you may want to read over as well:

https://www.digitalocean.com/community/questions/how-should-i-implement-proxying-and-redirects-on-the-app-platform

Let me know if you have any further questions!