You're still biting the bullet? Then lets do it. Replace the fat man with farmed babies. Doesn't satisfy the secret condition but since only farmed babies are used, no one has to worry about getting kidnapped and having their organs harvested - so no worries about society collapsing into anarchy over fear. The utilitarian credentials of this world are impeccable - the loss of utility from killing farmed humans is way overshadowed by the advances in biomedical technology and research.

If you're still willing to bite this bullet, I'm curious what you would say about my other objection. Even if utilitarianism is accepted as a global theory of ethics (evaluating what worlds are better than others), no utilitarian is self consistent enough to apply this theory to himself/herself. Does anyone actually think that the morally ideal life is one spent barely subsisting while focusing almost all of their efforts on others? Its quite easy to say yes from then comfort of your current life, to affirm a belief in belief, but to actually believe in this ideal and try to live up to it is much harder - not even Peter Singer comes close.