When my husband worked at the Integral Urban House in Berkeley, CA, about 25 years ago, they had a composting toilet that was designed to support a family of four. It took three years for the contents to compost completely, and the resulting fertilizer could only be used on trees or non-food crops. As Dick says, it doesn't seem like it would work for something short-term like a festival.

The city of Milwaukee used to sell a fertilizer called Milorganite, made from the finest products of Milwaukee's sewers, suitably composted and sterilized. I don't know if it's still available or not, but I believe it was also recommended for things like lawns.