I think what he means by "smart" motion blur is only certain objects have blur on them. Say, for example, high velocity objects. Only objects moving at high speeds could be blurred, like when you launch a body at a planet or moon.
I'd say, if you were to add motion blur, have it be very limited in what is blurred. It might be hard, though, because in such the vast emptiness of space, rendering motion blur on objects will be hard to see. The scale at which Universe Sandbox works on, you won't really be able to see it once you zoom out. It's a cool idea, just not one that would have any practical use for a simulator like this.