Just ask your Travel Agent to put in a request. If you're not using a travel agent, you can simply call the DCL phone number and ask a reservation agent to put in the request. They will not promise anything, but at least they may try to get you in that Head Server's dining rotation. If they're unable to do that for you, you can always try to make dining changes when you first board the ship. (Rockin' Bar D, on the Magic; D-Lounge on the Fantasy/Dream).
Funny story, which some of you already know. One of our favorite DCL servers is Kendell. We first had him as an assistant server on the Magic in '07 for our Med cruise. When we learned that he had moved to the Dream as a server, we requested him again. They were able to put us in his section and it was wonderful to be with him again. But the funny part is that he had actually been promoted again to Head Server. The only reason that we were able to have him as a server is that Karl Holz (President of DCL) was on our same cruise and asked specifically for Kendell as his server. So for one 4-night cruise, Kendell waited tables again, and that just happened to be the cruise we were on. So we got to sit at the next table over from Karl and his family.