When I use WatiN to go to a specific web page, how can I fake the HTTP referrer with a query string (i.e. request is from google search wit
I might be missing the point of what you're trying to do, but I don't think WatiN is capable of fakingbthe referrer.
WatiN drives real browsers, so it isn't in the job of faking stuff. If you want a page to have a specific referrer then you need to have it linked from that referrer.
As you suggest, Fiddler is much better suited for this sort of thing. If you want you can modify requests on the fly using custom rules so that the referrer is whatever you like. You don't need to use Fiddlercore for this. Running Fiddler as a client proxy, or on you server as a reverse proxy might be enough.