The movie, which also stars Stiller as struggling action star Tugg Speedman and Jack Black as gross-out king Jeff Portnoy, features some testy altercations between Downey's Lazarus and Jackson's Chino.
While the Barbarians await replies from those star names, Llanelli Scarlets once-capped All Blacks centre Regan King has accepted his chance to play for the Barbarians.
Heart, Randy Newman, Public Enemy and Rush are expected to perform at Thursday's star-studded event, while Usher will sing for Jones, Carole King will perform for Adler and John Mayer and Gary Clark Jr. will pay tribute to the late King.