I think what you may need to do is extend from Extension, not DataObjectDecorator. Also, I notice you've called it a PodcastController. Semantically speaking, this doesn't make sense. It would probably be something more like PodcastPageExtension.
Try defining onBeforeInit() or onBeforeInit() on your extension class that is applied to Page_Controller.
I think onBeforeInit() and onAfterInit() are only in 2.4. However, you could apply this patch if using SS 2.3: [url]http://open.silverstripe.org/changeset/78035/modules/sapphire/trunk/core/control/Controller.php[/url]
In your _config.php, do the following instead as well:
Just out of interest, what are you trying to achieve with the extension to Page_Controller?