With all this talk of ejectable boxex ect ect would it not be an easier option to make the batteries in the pinger last longer thus helping locate the recorders?
Or even enclose another pinger inside some material that breaks down over time in salt water (say 30 days) and then as the enclosed pinger gets wet it starts broadcasting? You could do this a couple of times with varying degrees of material so that as each pinger dies the next one starts broadcasting.
just a thought.