I've heard about these, I have friends in West Texas who use a composting toilet. The Texas State Parks have installed them in places like Enchanted Rock where they don't have the ability to set up a water flush system in the park.
Other countries are way ahead of us on this, I think. Japan comes to mind, but I don't have any citations to offer you at the moment. Unfortunately, there is an "ick factor," a species bias (somehow human poo is more toxic than animal) and I suppose if there are drugs in the system there may be some residual chemical trace, as is found in waste water plants.
It bears consideration, however. And you might also want to consider the
toilet paper end of things as well.