@IanW
No doubt the transponders that have a potential battery life of 10 years are of a similar nature to those used on diving bells. Rather than "pinging" relentlessly from day one until exhausted, they save their power to reply to interrogative pings from searchers, allowing the SAR team the time to determine and arrive at the search area.
Having such reactive ULBs attached to the FDR & CVR seems like a good idea, but for (rare!) scenarios such as this one another transponder producing a lower frequency signal with a much greater range would also be damned useful. It wouldn't have to be associated with black boxes and could at least alert the searchers to the general location of the airframe.