Maybe a really short film, as part of the virals for people waiting for B3. Maybe afterwards, by a director hoping to take the helm after Nolan- just to stamp his signature style on a product in the same universe.
In this Batman realm, Amadeus has yet to even be mentioned once. Since Batman Begins, Arkham wasn't even really referenced- so unless it's a main setting for the next film, it wouldn't really matter much or add to the proceedings.
I'd say if anything- do an origin story on Alfred, do a mock-up 'History of Gotham' for the History Channel, or more episodes of Gotham Tonight with Mike Engel-- featuring special guest Edward Nigma.
I really think that while Amadeus had a very interesting backstory and life- as alluded to in the Arkham Asylum game, most non-hardcore Batfans just won't give two shakes of a mallard's tale.