Overwriting Enclosure URL

Have created a podcast content type using local file storage on the server. Standard Audio element. Setting up the podcast feed normally works well and the podcast feeds successfully to iTunes. Want to override the enclosure url to send the request through a tracking service like podtrac.

Normal Conditions - Works now.

Would like to have:

Rewrite results on the content type doesn't work (since I gather the formatter is a composite call.) Is there a clean way to reformat the url?