gistfile1.js

var _payloadArgs = null;
function installMonitor(){

    var rootElement = document.body || document.head || document.documentElement;

    // Create communications dom elem
    var div = $('<div/>').attr({"id": "unique"}).css({display: "none"});
    div.appendTo(mainElement);

    // this is our click handler that the injected js will call.
    div[0].onclick = function(){
        // Save the vars and the interval below will pick them up.
        _payloadVars = [this.args];
    };

    // actually request the payload when the _payload vars are set.
    var interval = setInterval(function(){
        if(_payloadArgs){
            getPayload(_payloadArgs, function(payload){
                MYOBJECT.doSomething(payload, _payloadArgs);
            });
            clearInterval(interval);
        }
    }, 100);

    var script = $("<script />");
    script.text("(" + __monitorForPayload + ")();");
    script.appendTo(rootElement);
}

// Function injected into the actual page
function __monitorForPayload(){
    var oldCallback = null; // old version of the function

    // The pixel payload will call this function
    var callback = function (args){
        // create the new div, set it's attributes and values to pick up later
        var newSpan = $("<span />").attr({"type": "hidden",
                                          "id": "unique-2",
                                          "args": "some-variable"});
        newSpan.insertBefore(document.body.firstChild);

        // Make the callback to the contentscript's fn.
        var elem = document.getElementById("unique");
        if (elem && typeof elem.onclick == "function") {
            elem.args = "some-variable"; //ghetto
            elem.onclick.apply(elem);
        }
        return oldSetPC.call(this, args);
    };

    // We dont know when the object will be available, so we poll.
    var interval = setInterval(function(){
        if(window.__some_class){
            oldCallback = __some_class.callback;
            __some_class.callback = callback;
            clearInterval(interval);
        }
    }, 20);
}