Here is a bit of idle speculation.
It regards the pingers. Yes, the batteries are deader than dead by now.
The actual oscillating element on the pingers must have a characteristic frequency (by oscillating element, I mean the item that transmits the vibration to the water.)
If you could excite this with a pulse of the appropriate frequency, and if it were nearby, you might receive a slight signal back from the oscillating element after the exciting signal cut off. This signal would have a characteristic amplitude decay over time which should be easy to characterize from experiment with similar pingers. You would also receive echos of the original pulse as it bounced off more distant items, but these would not have the characteristic decay signature.
You might also be able to transmit an odd harmonic and receive back some primary signal, but my math is a bit stale in this regard.
With the number of EEs looking at this thread, I'm sure the darts will be flying if it won't work. Fire when ready.
If something like this would work, it might help find pingers lightly buried in silt or beneath other objects. On AF447, I would be most surprised if the pingers are still attached to the memory modules.