This is a JS snippet similar to one that I wrote for an optimizely experiment that needed up update button text everytime a new set of products on our site was fetched. The markup would render and then my callback would replace the text for the users in the experiment.

globally-responding-to-async-requests.js

yourCb = function(){
  console.log('hello world');
};

jQuery(function(){
  var w$ = window.$;
  var oldAjax = w$.ajax;
  w$.ajax = function(settings){
    var url = (typeof settings === "string") && settings;
    var success = null;
    if (url) {
      settings = {url:url};
    } else {
      success = settings.success;
    }
    settings.success = function(){
      success && success.apply(null, arguments);
      yourCb();
    };
    oldAjax(settings);
  };
});