Hi, I'm the guy who built the baths on https://rocketstovestuff.wordpress.com/
It's been nearly 3 years since I started experimenting with this concept and have come to the same conclusion as Erica, that the primary heat source ideally should be solar or better yet a compost hot water heater, like the one developed by Jean Pan. The rocket is there for a booster to maintain the desired temperature as this varies amongst people.
I got the idea from Ianto Evans book, just that little diagram and one paragraph near the back. I also worked with Tim Barker in New Zealand for a few weeks on and off as he was coming back and forth from PRI in NSW whilst I was doing a 6 month internship in building and appropriate technology. That's where I learnt how to build the stoves and I'd say I learnt the most from Tim.
In reply to a few comments previous, I felt the experience of bathing under the stars was a one up on inside (personal preference), so that was the whole point in the 'outside' bath. Also I appreciate the idea of having a bell under the bath but it would make it a bit tricky to load the feed chamber when your sitting in it, especially if the bath is built up on four walls. I have built them on slopes which is great as the stove sits downhill of the bath but on flat sites I just build a metal frame and lift the whole tub up by 4-500 mm. This is a good height to still be able to drop more wood in the feed chamber. To get around burning your bum I simply cut a piece of wood to fit the inside of the bath, it's appropriate, easy and very comfortable, especially if you put a yoga mat on top of that as well! I place a yoga mat on top of the water whilst heating also. The first two I made were cobbed and this definitely slowed down the heating. Now I don't use cob, but a light earth mix (insulation) of straw and clay slip to about 3-4 inches thick all around the bath. Now they heat up in 30 - 40 minutes. Marion is right in saying that we just want hot bath water, not scalding for cooking. And yes you could just dig a hole in a bank but I felt like making something that looked nice as well as providing basic function.
If it's not appropriate to your environment and personal context, find out what is and do just that. I've had a lot of fun experimenting and hope to keep building beautiful baths =)
Thanks for all the interest, advice and support.
Remember to find a bathing buddy, it saves water