property_override.js

var tabGroup = Ti.UI.createTabGroup();

var tabsArray = tabGroup.tabs;

Object.defineProperty(
    tabGroup, 
    "tabs", 
    {
        get: function () {
            alert(tabGroup.tabs.length + " tabs");
            return tabsArray;
        },
        set: function (tabs) {
            tabsArray = tabs;
        }
    }
);


// or even

tabGroup.addTab = function (tab) {
    tab.icon = ""; // do funky things with our tab
    Ti.UI.TabGroup.prototype.addTab.call(tabGroup, tab);
};