If you've ever read "A diet for a small planet" or other books which advocate vegetarianism on the grounds that it is the best way to feed all earth's citizens (minimizing energy losses from eating other top-of-the-food-chain animals like beef and tuna). The problem with our current burial practices, is the same. Except this time we are the top-of-the food chain animal, and we are removing all the nutrients and energy that we've accumulated in our bodies from the biocycle. These ideas you point out are really exciting.
A slightly less exciting, but nevertheless interesting idea is the one being pursued by Eternal reefs (www.eternalreefs.com) which provides the option for your cremated remains to become part of a constructed coral reef. In this case, the physical remains are pretty much the same as any cremated remains -- wasting energy and fossil fuels. The difference is that they leverage the desire for that person's remains to live on into funding for reviving reef habitat. And, it's significantly easier, because cremation, as opposed to composting, is already socially accepted.
Maybe other concepts based on this approach could be used to stepping stone into an entirely green method for burial.