rcsa,
Like several other people on this thread, you are proposing a very specific, and hideously expensive, "solution", to an extremely rare problem.
A similar accident may not happen again for years and years, and when it happens, it won't be identical.... maybe the aircraft ditches, "digs in", turns upside down, and sinks more or less intact, with your "floating FDR" floating up inside the tail, and still ending up 4000m down......
And why a second FDR? Why, in that case, not modify the existing FDR?
Try to think your suggestion, through, fully. If the original FDR didn't break loose from the wreckage, why should another one do so?
In the case of AF447 there were debris, some big ones, on the surface. Even after trying to plot the currents, there still is no clue to the actual location of the debris field on the bottom.
Some simple suggestions in this thread may make sense.
Pockets of dye marker spread through the aircraft might be a help. Maybe one should go into each life vest?
And contrary to radio-active tracers, they would need no other equipment in the first search aircraft on the spot except the Mk 1 eyeball.
Updating the specs of the "pingers" in the recorders might help as well.
'Crying for help' a couple of days, then go to 'listen and reply' mode, with modern technology in the same unit housing, should be feasible.
We should not try to think of esoteric solutions to an exotic accident, but we should try to learn the lessons.
If some simple action can solve a future accident like this, and at the same time, improve search and rescue, and recovery of the flight recorders, in generally similar occasions, the industry will probably end up implementing it.
CJ