getClassByLoader.as

var loader:Loader = new Loader();

var request:URLRequest = new URLRequest('YOUR_SWF_PATH');
loader = new Loader();
loader.load(request);
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, onLoaderComplete);

private function onLoaderComplete(event:Event):void
{
	var loader:LoaderInfo = event.currentTarget as LoaderInfo;
	loader.removeEventListener(Event.COMPLETE, onLoaderComplete);
}

private function getClassReference(className:String):Class
{
	return loader.contentLoaderInfo.getDefinition(className) as Class;
}