You are allowed to build and prepare before hand. The Idea is to get a bunch of smart interesting people together in a room, make cool thing and have fun. I would guess by looking at some of the ideas so far that the hackathon will be just a part of some designs that could becomes companies. Others will be really great projects that will be done almost entirely in the event. Do you have an idea of what you want to work on?
Yes you can participate on your own. We have found that most of the time groups do better, but there are one person armies also work sometimes!
We will not out you either, we are going to be talking with all the teams and make sure that we can talk about what you are working on in any of the press that we do, we know that sometimes you have to keep quiet for a little bit.