I did a little research into composting toilets the last few days. The ones you can buy are, apparently, rubbish. I read review after review about several brands of them, and 99% of the reviewers had nothing good to say about them. They cost well over $1,000 and have complicated drum and rake systems. Then I found a link to The Humanure Handbook, and found my answer. This is an utterly simple system for a composting toilet. A box, a bucket, some sawdust, and seat. And it doesn't waste a gallon and a half of perfectly good drinking water with every use, since there is no water required and no flushing.
They sell these very reasonably, but I can't see any reason to buy one when they are so simple to make. There are some great pictures on that site of toilets made by the readers of the book, and some are very, very nice! The only other point to using one of these is that you need a compost bin, or a service to haul away the "produce." I'm pretty well sold on this composting toilet, but I wonder how accepted they are by local health departments. Will I still be required to have a septic system just for the grey water produced by my shower and sink? That I will have to find out.
On another note, I finally came up with an acceptable floor plan for a 12x18 cabin, which is as small as I can go to get in all my requirements for space and storage capacity. Yay!