AN IFR FLIGHT WAS TERMINATED WITH A VISUAL APPROACH. CONDITIONS WERE CONDUCIVE TO AIRFRAME ICING. THE AIRCRAFT WAS BEING POSITIONED EMPTY, WITH A CG AT 22.1% MAC. ON SHORT FINAL, AT APPROXIMATELY 400 FT AGL, 35 DEGREES OF LANDING FLAP WAS SELECTED. THE AIRCRAFT WAS OBSERVED TO PITCH DOWNWARD TO AN UNUSUAL ATTITUDE AND TO ENTER A STEEP DESCENT. A PARTIAL RECOVERY WAS OBSERVED BEFORE THE AIRCRAFT IMPACTED A DIRT HILL 500 FT SHORT OF RWY 28. EXAMINATION OF THE AIRFRAME AFTER THE ACCIDENT REVEALED 1/2 TO 3/4 INCH OF RIME ICE ADHERING TO THE LEADING EDGE OF THE HORIZONTAL STABILIZER. NO ICE WAS FOUND ON ANY OTHER PORTION OF THE AIRFRAME. EVIDENCE IN THE COCKPIT INDICATED THAT ENGINE, PITOT, AND WINDSHIELD ANTI-ICE SYSTEMS WERE ON, BUT WING/EMPENNAGE DEICE WAS OFF. NO EVIDENCE OF A POWERPLANT OR SYSTEMS MALFUNCTION WAS FOUND.