Other players will be announced in the coming days, but I'm not going to stay up late looking for the announcements. I'm guessing Mariano Rivera will wear #42 that day for the Yankees.
Players I'm guessing would also wear #42 would include Preston Wilson, Dontrelle Willis, and C.C. Sabathia, all of whom have been speaking out recently about opportunities for African-Americans in baseball. Sabathia later asked Barfield to wear #42 since he was not scheduled to pitch that day and Sabathia felt it should be displayed on the field, see the link above. Presently, the Braves, Rockies, and Astros have no African-American players on their 25-man roster.