I dont know if boosting will be restricted but they have nerfed the spells and dungeons against boosting so mages will no longer be able to boost. Maybe that paladins can boost but since they not around yet on horde i doubt that they are gonna be able to boost before 70.
One thing that maybe could be possible is to join a 5stack dungeon group that carry you in 5 man dungeons but u doubt it will be possible at launch, and probobly super expensive since its 4 ppl needed.
But i would not count on any boost tbh, best have your mind set to lvl uour own char i think.