adoiyaircraft:
Nice idea, but it won't work. You need somone to receive the sonar data you've transmitted. It will be detectable at about the same range as the pinger, or (probably) a shorter range. So if we can't find the pinger, we certainly can't get the data stream. (I design systems of this type for a living, BTW).
Not sure what you mean by RF transmission - the ACARS is the RF tx we use right now. Or do you mean a panic-mode download of data after an incident has started? Often that won't work (because the aircraft is in bits by the time you have data to send).
'Hard cases make bad law' as they say - the AF447 is just about the worst possible case - very deep water, Southern hemisphere (fewer deepwater resources available), developing nation involved, poor position reporting...
I'd like the pingers to use a lower frequency (to give longer range - maybe 5kHz rather than 37.5kHz?), and save power by transmitting only 1/4 of the time (or at 1/4 the ping rate) - this would give 4 months to get resources on site to do detection/recovery. (...But I'm guessing that nobody cares what I think about that...)
The deepwater ROV/salvage capability exists to recover wreckage and recorders, if someone's willing to foot the bill. AF447 can almost certainly be found, too, even without the pingers, if someone's willing to pay. Lots of people do very deep water (ie full ocean depth, 6km) sidescan sonar searches for shipwrecks. The precision survey work done in deep water for the oil and gas industry would blow your mind. Finding an aircraft or fragments of one can be done using this approach - but it takes more $$ the smaller the pieces you're looking for.
Regarding ejectable devices - we'd have to be super-duper confident that in the worst case (ie the AF crash) the device would be ejected successfully. If not, you've spent a bunch of money equipping the fleet with something which has contributed nothing to safety. The ejectable device would be required (ie black boxes would not be recoverable) maybe once every 20 years?
I'd prefer to see money spent on getting realtime satellite imagery into the cockpit, so that no plane I'm ever on goes anywhere near any thunderstorms.