the best analysis I've seen of chairs and seated posture is a book called
The Chair: Rethinking Culture, Body, and Design by Galen Cranz:
http://www.amazon.com/gp/search/ref=br_ss_hs/104-4832743-0742322?search-alias=aps&keywords=the chair
Yeah, there's no back support in a kneeling chair, because you don't need it. Look around some time. When people lean back in a chair, using the "back support" they're not using "good posture":
Their back is slumped in a C curve. Their cervical spine (their neck) is ratcheted forward, causing way more muscular tonus in the back of the neck than is necessary, depressing the sternum and limiting mobility in the upper ribs. And their lumbar vertabrae are rounded instead of arched. Just 'cause a back support is shaped like an "ideal lower spine shape" doesn't mean it supports the back in this way.
The trick of a kneeling chair is that it maintains an oblique angle between the thighs and the torso. This allows the lower back to arch the "natural way" (lordosis). (Saddle chairs acheive roughly the same thing, but without the strain on the knees).
By contrast, traditional chairs, which more or less maintain a 90 degree angle betweent the thighs and torso, prevent the lower back from arching. Sticking a curved back support onto the chair doesn't change this. (See picture below)
Lower back pain is epidemic in industrialized societies, but not in cultures where kneeling and squatting are commonplace.
BTW, there are some interesting product from this site:
http://www.officeorganix.com
or find a good practitioner of the Feldenkrais Method or the Alexander Technique.