I'm actually going to take a guess and say that it's on one of the extended edition disks for Two Towers or Return of the King. It's a flashback if I remember correctly.
As far as I know, taping of The Hobbit hasn't even begun yet and as far as IMDB goes, the only cast that has been confirmed is Hugo Weaving (Elrond), Ian McKellen (Gandalf), and Andy Serkis (Gollum). I don't even think that Boromir would have been born yet during the events of the Hobbit.