Burning Man and other outdoor gatherings have some commonalities: preparation, not all as ad hoc as they make out. Organizing committees, lessons learned from prior gatherings, assessing the terrain, planning for the number of people attending. Its all doable, as any event organizer can tell you - a safe number of EMTs to have on hand per thousand of attendees, how fast can a port a potty fill before it gets emptied, food and food preparers. I know one EMT who went down to do medical response at Burning Man the past couple years. Another friend of mine likes to go and cook. Most of the basic preparations happen as planned, otherwise there would be headlines.
The Rainbow Gathering is another moveable feast of a few thousand folks who choose an outdoor environment and camp out there annually. They rely on volunteers to do the heavy lifting, food, shelter, medical work etc etc. It all tends to come together, although I'm sure no one is prepared as possible for a sudden windstorm, lightning strikes, snow in July, lahar etc. Generally speaking in the event of a disaster I'm certain most people would just start bugging out, or stay put. Every year there's a celebrated child birth or two, every year there's also a death or two in the camp, usually from natural causes - that's just the actuaries talking, at any given moment a few of us are going to die, and some of us may die during a Rainbow moment. This year I think there were two deaths, and at least one person gone missing, although the circumstances of the disappearance weren't clear, it may be the case of someone just deciding to go walkabout after the gathering.