I asked about composting heads on another list that I am on and got this response. This is weekend and short cruise use, not live aboard.
http://www.natureshead.net/I Installed a Nature's Head a few months ago on my bubbletop 34 and I absolutely love it. I used to have to always do maintenance, futz with the flushing, my holding tank overflowed when guests overpumped, and the smell was penetrating all the hoses. Now it's so simple. I was able to vent it easily by cutting the pumpout hose a couple of feet below the outlet on the deck and splice it to the venthose coming from the toilet. On the topside, I simply removed the threaded cap from the pumpout outlet and replaced it with a threaded PVC fitting (exact same thread - I think it was a 1.5 inch NPT) and easily torqued it to seal. Then I added two more elbows to make it an inverted U so water would not enter from the top either. The only other think I had to do was to raise, by an inch and cantilever forward by about 4 or 5 inches the platform where the former head had sat. That gave me perfect positioning. It IS, however a little higher than a normal toilet because of the 1 inch lift and the fact that the new toilet, out of the box is already a little higher than normal. So far, however, no one has had an issue with it and certainly not me.