Hmm, is this with one faction or the other, or both?
I'm on Empire, and finished Alderaan (the first time) at level 40-41. We'd actually cleared out a lot of the bonus-level heroics, but couldn't get the actual bonus storyline at that time. Once we cleared chapter 2, though (at level 50.. talk about overleveled), the Fleet NPC lit up.
(Going by the suggested levels, they certainly *intend* for you to do the Alde bonus after Hoth, but there's no real reason to prevent you from doing it early, really. The timeline in the brief bit of story you get on the Imperial side isn't that specific...)
Pretty sure I did it with my RT and SI and maybe some other ones as well.