Redirecting Request (nsiHttpChannel?) in Firefox Extensions

Question

I've been trying at this for a long time now, and no good results.

var myObserver = {
observe: function(subject, topic, data)
{
    if (topic == "http-on-examine-response") {
        //  implement later
    } else if(topic == "http-on-modify-request") {
        //  implement later
    }
},
QueryInterface : function (id)
{
    if (id.equals(Components.interfaces["nsIObserver"]) || id.equals(Components.interfaces["nsISupports"])) return this;
    throw Components.results.NS_NOINTERFACE;
}
};
var obs = new Service("observer-service", "ObserverService");
obs.addObserver(myObserver, "http-on-modify-request", false);

Basically, on "http-on-modify-request", I know how to examine the URI, figure out which window (if any) it's associated with, and a bunch of other stuff. What I can't figure out is how to redirect a request, which I know is possible from here, because I can get an nsIHttpChannel before any request is ever sent out.

Anyone know what to do? :/ I've been trying for a couple of weeks on and off, and got nowhere.