how to redirect (proxy) all requests for certain users

This might be the case of trying to be too clever. My overall goal is to create test accounts that I can use in production for our iOS app that speaks to an API (Node.js/Express). Eg: Think along these lines

if (isTestAccount(req.user)) return goToStagingServer(res)

Therefore, rather than pointing all requests in our TestFlight® to the staging API, I'd like to keep that (generally speaking) to the production API, and only re-route (talk to) Staging for certain users. Any ideas?