I wonder what sort of rights situation that would be, and (to put it more simply) if it would be allowed. Obviously RiffTrax comes to mind, with its downloadable commentaries by Mystery Science Theater 3000 alumni about current and classic blockbusters, and Aaron And Co. at Geek in the City do downloadable commentaries as well, but would it be considered "talking out of school" for just one of the credited writers of a film to talk about it in a non-official way like that? And would things be more complicated if, say, Whedon wanted to do a commentary on the film Speed, one of his uncredited re-writes? I suspect it would be complicated and not what the studios would want.
But one of Whedon's skills is explaining his work: there's a special pleasure to his commentaries, where I feel I learn a lot and get that knowledge from a thoughtful perspective. (And that reminds me: it's frustrating to me that as intelligent and thoughtful a filmmaker as Spike Lee apparently can't do a good commentary to save his life. Writing his films, writing his books about his films, and talking in interviews about them: those he's good at. Put him in a screening room with a microphone and a free copy of School Daze* and almost nothing of substance comes out.) And I like good explainers.
* In Living Color reference For The Win! Oh, except I probably can't say that my own comment is For The Win. Sorry; got excited.