I kinda thought that might a deterrent for the judges. What about a tiered system for competitions? Where the entrants have their brews judged in a separate class. Non-judges, judges, etc.
Since entrant classes are usually based on ability (like amateur vs pro), I think this would create the impression that judges are by definition better brewers than non-judges. This isn't necessarily true.
But there are some practical reasons not to worry. Rewards for judging are low - judges are spending money on gas/tolls to spend a long day thinking really critically about beer (not quite as much fun as you'd think) usually in exchange for lunch and an experience point. This tends to weed out anyone who doesn't have a highly altruistic sense of service to the homebrewing hobby.
Also, if someone wants to game the system, it's going to be really hard to stop them with rules. Again, we really depend on judges having a sense of service and honesty. And if you work at any competition of decent size, you'll get a sense that it would be hard to give a particular beer a boost. Out of hundreds of entries, a judge has a low chance of being on the flight with one particular entry. And that entry would still have to make it through mini-BOS and BOS to win.