I think you might misunderstand what happens at a hackathon. Usually participants show up with ideas for things to work on in their heads, or they join a project that someone else is working on while there.

There isn't anyone dictating what people work on, and no one has to "approve" their projects. It's functionally not that different from the participants doing their work by themselves at home. The only difference is a hackathon is an arbitrary time and place just for working on projects.