You'd have to be lucky enough to have a ray of light bouncing in the right position and direction, and you'd have to be clever enough to notice it at the right

You'd have to be lucky enough to have a ray of light bouncing in the right position and direction, and you'd have to be clever enough to notice it at the right moment (without slowing down regular light tracing algorithm by adding a billion comparison everywhere to figure out if it matches a motion vector). I don't want to be a naysayer after only considering it for two minutes but it doesn't sound very practical to me.