- You've already identified this, but as an applicant I want to know what exactly I am getting in return for my time. There are so many ways I could spend my time, including making money and progress on my projects, and being put into a room with 100 random people doesn't sound very useful. IMHO you need to define a clear goal that is shared by all participants. For example, finding a co-founder, getting feedback on a pitch, improving your public speaking, etc. However, once you are doing that you basically have a meetup/event that requires an application.
- The design school at Stanford (http://dschool.stanford.edu/) operates under a rather similar model. The school offer classes of varying duration (from days to month), each based on a different theme or project. All classes require students to apply in advance and describe their backgrounds. Participants are then chosen in a way to maximize the diversity of students from different departments/backgrounds. You may be able to steal some ideas by browsing around on the website.
- I believe there is an inverse relationship between the people you hope to attract and the people who want to attend such events. You are trying to be selective in that you are are choosing the "top applicants" (however you define that, probably based on past achievements in their statements). However, the successful people you hope to attract have very little incentive to attend such events because they already are successful, or have better options to spend their time.