Yep, just controlling the direction of the glow/rays and blurring them, masking them, but that's not what I'd consider a purely a "motion blur" per say, but similar in effect. I'd check CoreMelt or some others for a third party plugin to do this. The glow and rays and such inside of Motion are pretty limited. It is a neat trick. I'd look at third party plugins. Would be fun to figure it out.