I'm not entirely sure we will get to pick our own service tags. From what I saw, the service tags were all campaign characters' names, which suggests it refers to the you are using and therefore is pre-set. At any rate I doubt it will appear in multiplayer. It still has to work with the original Halo 3 remember.
That's not to say the original game couldn't be patched to utilise the new tags. I could be wrong. And if that is the case, I'm going with "JEFF".