THE ACFT WAS ON AN ILS APCH AT DUSK IN IMC & ICING CONDS WITH THE ALTERNATE AIR ON. ACCORDING TO THE PLT, HE HAD JUST DSCNDD BELOW AN OVC, ON FINAL APCH, WHEN THE ENG LOST PWR & WOULD NOT RESPOND TO THROTTLE APPLICATION. A FORCED LANDING WAS MADE IN A WOODED AREA SHORT OF THE RWY & THE ACFT WAS EXTENSIVELY DMGD. DRG AN EXAM OF THE ENG, THE TURBOCHARGER & WASTEGATE TRANSITION PIPES WERE FND SEPD AT THE FLANGE. CARBONACEOUS RESIDUE WAS FND IN THE AREA OF SEPN, BUT NO HEAT DAMAGE WAS NOTED. THE GASKET, BOLTS, WASHERS & SELF-LOCKING NUTS (FOR HOLDING THE PIPES TOGETHER) WERE NOT FND. LATER, THE ENG RAN SUCCESSFULLY DRG AN OPNL CHECK. DRG THE INVESTIGATION, 3 SIMILAR CASES OF PIPE SEPN WERE IDENTIFIED, WHICH INVOLVED ANOTHER ACFT; THESE RESULTED IN ONLY PARTIAL LOSSES OF PWR. THE RGT FRONT SEAT PAX RPRTD SEEING A TRACE OF ICE ON THE WINGS AS THE ACFT WAS DSCNDG THRU CLOUDS. THE PITOT HEAT, STALL WARNING HEAT, & ICE LIGHT SWITCH WERE FND IN THE 'OFF' POSITION. AT 1650 CST, THE TEMP & DEW POINT AT THE SURFACE WERE 36 & 35 DEG, RESPECTIVELY.