I think the idea of EPIRBs is not the best way forward. I would think more about using the satellite broadband link. Audio feed parallel with a CVR channel, 48 kb codec (G722 or similar would give enough audio bandwidth to ID warning tones) add a 2kb slow data channel for an integrated GPS data. Add error correction and you could fit 2 into the minimum broadband link.
Activation could be for example an aircraft loses ADS-B, VHF comms, departs flight plan, departs cleared altitude etc then ATC could poll the aircraft and switch on the CVR link.
Automatic battery back up incase of power failure.
The issue will be with how those in the cockpit feel about possible remote monitoring of anything they say?