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.
out of curiosity, is there anything wrong with overriding index() instead of init()?
what i am doing is building a podcast module which essentially contains a dataobject that holds information for each episode, and gets attached to pages through a many_many relationship table. i considered making a podcast page, but i wanted to investigate extending pre-existing classes and overriding init() to add the podcast rss feed so itunes users can subscribe.