Just realized that an extension I am trying to do is architechturally similar to Ext.form.HtmlEditor, where the field is composed of an iframe element rather than a native form input field. In may case, I have a full-fledged component - a subclass of Ext.Panel (Ext.ux.ScreenshotUploaadPanel), and I wand it to 'act' as the 'input component'. Have not had any luck so far. Please see this thread for more info.