The fix though is easy.
In WCF tester, add your reference to your service, say http://localhost:12345/myservice.svc. Then, right click on the “Config File” node in WCF tester and select to edit with SvcConfigEditor. Find your endpoint under client -> endpoint, and then change http://localhost:12345/myservice.svc to http://localhost.fiddler:12345/myservice.svc and hey presto, the traffic gets pushed through fiddler.
You might wonder why I didn’t just add the service reference as http://localhost.fiddler:12345/myservice.svc - well, when I did that, any method invocations didn’t seem to trace in fiddler. May have been user error on my part, but the above worked for me and I could diagnose the raw SOAP messages.