Three Mile Island Camp has used a six hole Clivus Multrum setup for about a decade now with great success.
http://www.clivusmultrum.com/
I think that their offerings run more to the commercial/institutional side of things as opposed to residential, but they might be worth checking out. At the very least, I can tell you that the Town of Meredith allows composting toilets on island property. I can't remember what kind of state approval, if any, was necessary before installation.