How about just animating the Model of the tail of the trench coat, blowing in the "wind" (giving it a long sequence so it doesn't look looped in the game), then attach it to the body of your player (I'm no good at scripting but) somehow make it so that when you move foward to top of the tail peice is hinged to the player thus making the bottom of the coat slightly flare back in movement. You could even use a hinge from newton.
