"Reissue from composer" is a feature that takes the request from given session and let you create new request from the composer mimicking the original session. What you are trying to do by adding your code to "BeforeRequest" is to change all requests, whose URL is matching "original_url", which is totally different scenario.
If you are trying to change the outgoing sessions, then you are on the right track. If the server does not return any bytes, then maybe your request has something wrong. If you want to replicate the "Reissue from composer" you can use "Proxy.SendRequest" method to send request. Use already captured session to fill the arguments (if you want to fully replicate the "Reissue .." feature, if not you can create them yourself). Proxy instance can be accessed though FiddlerApplication.oProxy.
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