Having started this thread, it is apparent from subsequent comments that the idea is not completely impossible, but expensive to implement or to retrofit.
So I suggest a simplified, much cheaper idea (perhaps interim) with only one parameter in the telemetry, GPS location. If a GPS location becomes static or ceases completely, this MIGHT indicate a problem. Then in the event of an accident there would then at least be a narrower area in which to search for the FDR.
Very similar has already been used in gliding competitions. Telemetry from GPS is used to show competitors’ positions. In one recent contest in New Zealand a pilot crashed. Unfortunately, he was killed and the organisers didn’t at the time notice his stationary GPS readout. I had the benefit of hindsight when watching the replay already aware of the crash and noticed the stationary GPS. Software that detects stationary or loss of GPS would presumably be very simple.
Is this an impractical idea?
Jack