A question came up at the AskPhilosophers.org site recently, on the subject of ethical egoism and supererogation, which brought up an interesting point; Allen Stairs suggested in response that "If any ethical theory claims that we're obliged to maximize something or other, then there's no room left for supererogation." And, indeed, initially this would seem to be the case. However, it really only does so if we add to the maximization condition the further condition that nothing non-obligatory contributes to the value of what you do.
Suppose (for the sake of argument) that you are obliged to maximize the good of others; but suppose that there are other things beside the good of others that have value, e.g., your own good, to which we are not obliged. This allows room for supererogation, since we can go beyond our obligation with acts that are valuable but not obligated. I think this shows that the real issue is not maximization but exclusivity, because the claim that nothing non-obligatory is of (relevant) value suffices on its own to eliminate supererogation.
Usually when we talk about ethical egoism, the sort of textbook caricature introduced in introductory ethics courses, we are talking about something committed to an exclusivity claim: nothing other than self-interest contributes to the value of an act, and maximizing self-interest is what makes an act right. But it would seem doubtful to say that anyone tempted toward ethical egoism would actually commit to the exclusivity claim, unless he is conflating a sort of psychological egoism with his ethical egoism, and therefore making it impossible for there to be anything of value other than self-interest. So ethical egoists need not have any (direct) problem with supererogation, because they need not be committed to the exclusivity claim. (As Stairs notes, ethical egoists need not be committed to the maximization claim, either.)