The comparison between road and MTB bar width is pretty hard to apply; for one thing, the road bike averages about 200mm from the center of the stem to the hoods. On a MTB, it's barely more than half that. Add in the steering geometry differences, it's almost apples and oranges.

I've personally never seen a MTB handlebar shorter than 580mm. I've HEARD of flat-bar fixies as narrow was what you describe, but they don't see trails. You need the leverage of the wider bar for dirt.

I personally run 660 on my bike, and have considered 710. (Using the road-bike measurement method, shoulder-width, I'd need a 505mm drop bar.)