Two years later I finally revisited this, and got it figured out. Apparently if you have a MODX.Window with tabs, you need to cycle through each of the tabs on "activate", otherwise the elements aren't generated, and you have no 'dom' to load the DropZones into.
At the bottom of your window put:
test.window.Example.superclass.constructor.call(this,config);
this.on('activate',function() {
Ext.getCmp(config.record.window+'-second-tab').show();
Ext.getCmp(config.record.window+'-first-tab').show();
});
};
Adjusting the Ext.getCmp()'s to reflect the ID of each tab in reverse order.