I was wondering if there could be some sort of optional tickybox or author note where people could say if they gave permission for their fic to be podficced later on and what that permission was (blanket, with notification, etc.)

We aren't coding the sign-up form so I don't know about including a tickybox on it or even on the fanwork uploading/submission, but we'll definitely consider adding a suggestion to submission guidelines about adding an author's notes about giving permission to have their stories to be podficced if they chose.