I've been to a private gathering in a wood where we used an earth closet. It's fine for small gatherings in land not used for anything other than growing things.
Human excrement is not really suitable for 'fertilising' high use areas such as playing fields. It's something to do with the meat content of the diet. Omnivourous and herbivourous diets are OK, the vegetable matter breaks down and becomes good bacteria, but carnivourous poop has entirely the wrong sort of bacteria. If you have cats that poop in the garden, you'll know, there is a barren patch where they go and little piles of poop that don't break down for ages. And then there is the risk of disease from bad bacteria that haven't broken down. There are several illnesses that can be passed on through contact with various sorts of poop, so it's very important to have a deep enough drop and enough spare soil to put on top.
The right kind of paper must be used too.. The cheap, hard, shiny stuff (usually branded as Bronco or Izal) is not suitable. It doesn't absorb moisture and it doesn't degrade quickly enough. Tissue is better, but make sure it's a recycled, unbleached tissue - it won't have so many chemicals in it.
There are commercially available composting toilets, but I seems to remember it's all stored and taken away to be processed.... maybe it was just the brand I saw. They seem to do this sort of thing with less embarassment and more consideration in Europe.
On the whole, unless you have a sponsor who is willing to pay extra for composting toilets, it's not a good idea for a large amount of people to use them over a short period of time. Small gatherings in low traffic areas (arable or livestock farming, waste ground) are fine with a suitably built earth closet, but large gatherings need more and that starts to get difficult.
Another thing you'd have to take into consideration if you go for the composting variety, local health and safety by-laws. There are strict regulations for the managment of human waste. I suspect that the already more expensive composting toilets will suddenly become totally unaffordable once the local councils get involved... unless of course, you have one of these incredibly rare local councils that are eager to help people recycle and conserve.
Good luck with your search though... it's great that there are still people who think that far ahead about recycling and conservation.