It is not a new tradition to have a director have a cameo in their films.  You can find cameos of directors like Steven Spielberg, George Lucas, Martin Scorsese, Oliver Stone, Peter Jackson, and perhaps the cameo expert Alfred Hitchcock.

Composers themselves have made cameos in many films.  They typically range from conductor to musician.  There are a few instances of the composer appearing as themselves or in a speaking role.   
Here is my compiled list of some nice composer cameos.

JOHN BARRY

Barry conducts in Deadfall (1968)
John Barry in The Living Daylights (1987)
DAFT PUNK
Daft Punk in Tron: Legacy (2010)
DANNY ELFMAN
Danny Elfman as the Devil in Forbidden Zone (1982)
Danny Elfman in The Gift (2000)
MICHAEL GIACCHINO
Michael Giacchino in an episode of Undercovers (2010)
JERRY GOLDSMITH
Jerry Goldsmith at the piano in In Harm's Way (1965)

Goldsmith (man in hat) and Steven Spielberg in front in Gremlins (1984)
Jerry Goldsmith speaks in Gremlins 2 (1990)
BERNARD HERRMANN
Bernard Herrmann conducts in The Man Who Knew Too Much (1956)
JAMES HORNER
James Horner passes by the camera (right) in Star Trek II: Wrath of Khan (1982)
QUINCY JONES
Quincy Jones referenced by Austin Powers in Goldmember (2002)
HENRY MANCINI
Henry Mancini hands the baton to the Pink Panther in Son of the Pink Panther (1993)

ALFRED NEWMAN
Alfred Newman conducts Jascha Heifetz in They Shall Have Music (1939)
Alfred Newman conducts in How to Marry a Millionaire (1953)
DAVID NEWMAN
David Newman (center) in The Runestone (1991)
MIKLÓS RÓZSA
Rozsa conducts in The Private Life of Sherlock Holmes (1970)

LALO SCHIFRIN
Lalo Shifrin conducts in Red Dragon (2002)
MARC SHAIMAN
Marc Shaiman (R) with composer Glen Roven in Broadcast News (1987)

Marc Shaiman (sorta) in South Park: Bigger Longer & Uncut (1999)
HOWARD SHORE
Howard Shore (left) in The Lord of the Rings: Return of the King (Extended Edition)
Howard Shore conducts in King Kong (2005)
JOHN WILLIAMS
John Williams (left) plays piano in an episode of Johnny Staccato (1959)
CHRISTOPHER YOUNG
Christopher Young (left) in Spider-man 3 (2007)

Any other composer cameos you know about that didn't make the list?