I think this feature request will become more relevant everyday with the current uprise in Single Page Applications (SPAs).
The purpose of a reverse proxy in such a scenario is quite simple;
Lets say for example, you have a SPA running on localhost:8080, you may want to point any /api/ requests to either localhost:8100/api for local development or to mytestserver.com/api for test development.
I would suggest such a feature would have its own tab in the settings for the site / app /service
I tried to upload an image, but the button doesnt seem to work.