Doesn't sound that good for the back IMO, as someone who can squat indefinitely it's not that comfortable leaning forwards like I do on a desk. You end up hunching up even more, and your knees are just in the way of your arms.
Kneeling is pain though. I'd rather sit cross legged than kneel for any length of time. A cross legged desk would work, as well. It would just be about a foot high.
You could probably use a low Japanese table for this pretty well.
I think a combination of positions is probably best to get variety in muscles you're using. Reminds me of a theory I heard that hard-backed chairs are actually better because they make you fidget and readjust your body, versus using the soft chair as an external spine
Kneeling is pain though. I'd rather sit cross legged than kneel for any length of time. A cross legged desk would work, as well. It would just be about a foot high.