With regard to powering the FDR when all engines are inoperative, remember you need more power than that required to run only the FDR. For the FDR data to be useful, you must also power all the sensors used to supply the FDR with data to record.
Having both engines inoperative puts a premium on available electrical power. Battery and/or RAT generated electrical power is devoted to essential flight systems needed to maintain control of the aircraft in a potential IFR environment.
A fully functional FDR would require a significantly larger set of batteries or larger/multiple RAT's. The airframe builders have been unable to see this as a good trade in terms of the increased equipment, weight and maintenance costs. After all, although the Air Transat dead stick landing was a significant piece of airmanship, the data necessary to understand the incident was recorded before the engines quit.