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.