hmm not from the crypto-community, but .. find the passengers that are comfortable sharing their weight; gather those, mark them in the total list with a tag, a certainty estimate from 0.0 to 1.0, and a weight. Now use the rest of the list to estimate weights based on similarity, assigning some reasonable certainty; show the other passenger those results, tell them again how the group benefits, and they personally benefit; iterate.. If an individual does not want to publicly share their weight, then show them how they can enter the weight without being named to the group. Iterate that, with their consent.

The reason I say this is because you are not talking to machine parts or to secret financial players, in this problem, to you are talking to human beings. By reducing human beings to machine parts to solve a problem, you are losing information that is critical to your own humanity ! and to theirs ! super-bad results can happen when people in power and their adjuncts, are successfully rewarded for reducing human beings to machine parts in their solutions ! Junior programmers finish your studies, but all humans please take note -- no cryptography required here !