PDR1
Unless I'm missing something this would not have helped in the MH370 scenario. The "distress circumstances" would either have to be based on some analysis of aircraft systems data or be crew-selected. From what we know of the MH370 case the aircraft systems would not have seen anything that looked like a "distress circumstance" unless the aeroplane actually ran out of fuel, and in the case of a deliberate "suicide" action the pilot would be unlikely to switch it on. It might help in a hijack scenario, but hijackers would probably soon learn how the system is switched off.
If mandatory cooperative surveillance systems such as SSR or ADS-B are both switched off while the aircraft is airborne it could be counted as 'distress circumstances'. Certainly, excessive cabin altitude would also be a distress circumstance. The system would then autonomously start transmitting position and if bandwidth permitted CVR/FDR without input from the crew and regardless of input from 'the crew'/hijackers.