That's not really possible with iMovie. You would need something that allows either keyframe animation or motion tracking. Final Cut Pro or After Effects (respectively) would do this for you.
You can use the "Peephole" plugin under the Editing->"VideoFX" section. Simply make Color 1 be the color you want to highlight with ( the darker the color the more opaque it is ) and make color2 be white ( transparent ) and move the little yellow dot on the video clip to where you want the effect to go.
Now an important thing is that you can't TRACK someone with this method. That is not really doable with iMovie... you'll need at least Final Cut Express HD for that. ( Or any version of Final Cut Pro )