I agree with Old Aero Guy, the high priority use of available electrical power after all engines are shut down, must be given to the critical systems and instruments that help the pilot get the aircraft down safely. All other priorities for use of the limited standby electrical power are secondary.
After thinking about this for a while, perhaps the current methods of supplying power to the FDR and CVR are correct just as they are. If the CVR unit in this aircraft has a 30 minute tape (correct me if I'm wrong about this), then the CVR stopped at just the right moment when the left engine flamed out. As others have pointed out, the events leading up to the fuel starvation of both engines, are far more important to the investigators that what happened afterwards. Because the CVR stopped when the left engine flamed out, the maximum amount of CVR data relevant to the events leading up to the fuel starvation, were preserved. BTW, I think that if anyone wants to know how an A330 performs in a glide, flight tests should be scheduled, which would provide a far better test data set wouldn't they?
To me, the most important question after what caused the fuel leak (which is already known), was what events lead to the premature fuel starvation of the left engine. This question is very critical regarding this incident, as a future event of this kind could send an aircraft into the water. The investigation may or may not implicate the pilots, but a thorough understanding of those events is crucial.
As the events unfolded, and with the FDR and CVR particularly stopping when they did, the maximum amount of data was preserved for the most critical part of this investigation. Perhaps nothing needs to be changed in the power sources of the FDR/CVR.