Cooking a roast in a dutch oven has been my holy grail. I've also been captivated by the idea of cooking it packed in rock salt - how does that change the roast? Does it help turn out a better roast? How salty is the result? I'm glad to have finally answered these questions.
I followed the directions from here roughly following the directions and some other tips I've picked up on the dutch oven Yahoo mailing list. I got a standard couple-pound pot roast from the grocery and cooked it in rock salt. It took 2.5 boxes to surround the roast in my 12 incher. I let it cook for the three hours recommended in the blog post with a 12/10 coal split and 6/4 coal replenishment every hour. This is where the plan fell apart: my internal sense of roast sizes did not flag that the massive five-bone rib roast in the blog post is much larger than the pot roast I get at the store. I really only needed about 2 hours of cooking for a nice medium rare roast. The extra hour pushed me to medium done. Getting the roast out was interesting - I had been told that the salt forms a shell around the roast and was eager to see what that looks like. In my case the shell was reasonably tough - resistant to stabbing with most tools - but I was still able to get the meat out. The roast shrunk about an inch in size creating an air gap on the inside of the shell which was probably due to the extra cooking hour it had. The salt can't really be recovered for later roasts.
Aside from the extra doneness the roast was a success. You do need to scrub the meat a bit to get all of the salt off - leaving a single salt chunk will throw the taste off so you need to get everything. Hopefully with less cooking time it is possible to portably cook a roast consistently.
You can see the air gap between the shell and the roast. The top, loose salt has been scraped away.