How to Change host for XHR and Scripts within Sites and redirect output to an Iframe

I need help on a best approach to proceed.

I need to build a simple web application where in I can pass a website url. And modify a host entry for one of the URIs to another server. For example its like making a hostfile change like 122.123.124.232 newhost.com

Later with this revised host map redirect all the sites output to an iframe within my webapplication.

The use case is for testing a tag management system in production and I would like to redirect requests to my test server with an online utility rather than the cumbersome need to change my hostfile or install a plugin in my browser to do so.

Please help me with some approaches on how to proceed. I was thinking a rails app with some watir or selenium not sure how though

Cheers!