Still Another angle
I am not talking about the CVR/FDR transmitting upon impact. What I am suggesting is that once the Officials reach the sight, they can transmit a "Trigger" signal that only the target CVR/FDR pair will respond to. If the Airplane is in deep water, there will be ships searching for the beacon. Once they locate the beacon, they transmit the command to the CVR/FDR to begin the ultrasonic data transmission, and collect the data from aboard ship. If an airplane crashes in the Andes, or some unreachable location, a search aircraft, flying overhead, could transmit the download command for the CVR/FDR to transmit the data over a radio frequency that then would be collected from the air. The trigger codes would be unique to individual FDR/CVR and would be maintained in the aircraft maintenence records or some other database.