Well, the good news is that some of the top dogs do this on the startline. Great article from Susan Garrett about her dog Buzz
In Defense of the Opinionated Dog | Susan Garrett's Dog Training Blog
Bretta will bark a bit on the startline when she's amped for a run (not always though) as well as sometimes on the table. Glory does it a bit.
Personally, you need to be careful about your criteria and training this away. Because you may THINK you are correcting/fixing the barking, but you may actually end up sucking the fun from agility with the dog just understanding that bad stuff happens at the startline. They may start jumping the startline, leaving the ring, just slowing down in general.
So figuring out another thing to train, or managing the startline (fast as possible), I know that some people have a kind of elaborate startline regime of having the dog go around and between their legs before the sit/stay then they immediately leave, set up and release the dogs. Seems like sometimes the more precise we are and more elaborate (but clear and fast) our routine is on the startline, the more it helps.
Additionally, very short leadouts at the beginning of this seem to help the most. Going too far too fast seems to really crank up some of our dogs and allow them to bark bark bark (and put it into their routine) and there's nothing to be done if we are on our way to a 3 jump leadout.
BTW, have you ever thought about attending any of Susan Garrett's training? She's used to high drive dogs (which, BTW many people do NOT have so they have no idea of the barking issue and what it really is and why you may (or may not) want to deal with it). The fact that you have a drivey dog that is loving agility is a GREAT thing and you do NOT want to lose that.