I can think of one scene that may be an example of the ego defense mechanism that you are looking for. There is a type of second election for chief in a meeting called by Jack. This meeting is found in the chapter entitled "Gift for the Darkness." Jack says, "Hands up, whoever wants Ralph not to be chief?" His question is met with silence. Jack's reaction comes in the form of "humiliating tears." He covers his pain by disappearing into the woods and saying, "I'm not going to play any longer. Not with you." When some of the boys find their way to Jack, he is "brilliantly happy."
Many of Jack's responses are a result of his trying to cover his embarrassment. When Ralph accuses him of letting the fire go out in Chapter 4, Jack responds that he killed a pig. When Ralph insists that Jack had let the fire go out, Jack in his anger, hits Piggy.
Jack uses escape, blame diversion, misplaced anger and violence as mechanism of coping with threats to his ego.
A good example of ego defense mechanisms takes place at the beginning of Chapter 10. In Chapter 9, the boys accidentally kill Simon when he stumbles into the middle of their savage frenzy on the beach. At the beginning of Chapter 10, Ralph mentions to Piggy that they murdered Simon. Piggy initially refuses to acknowledge Simon's death and utilizes the defense mechanisms of denial and repression. When Ralph says, "That was murder," Piggy responds by saying, "You stop it! . . . What good’re you doing talking like that? . . . It was dark. There was that—that bloody dance. There was lightning and thunder and rain. We was scared!" Denial is a defense mechanism that involves blocking out an unpleasant external event from awareness, and repression is when an individual keeps disturbing thoughts from becoming conscious. Piggy utilizes both defense mechanisms to avoid facing the reality that he participated in Simon's murder. Later on, Samneric return to Ralph's camp and also employ the same ego defense mechanisms. They also repress the horrific event of Simon's death and refuse to acknowledge that they were participants in his murder.