ULB
How much difference would it make to locating an ULB if the pulse rate was lower, or perhaps 1 pulse every 10 seconds for, say, five pulses then one every second for a further five pulses. The aim being to extend the battery life. Or even making pulses less and less frequent after 5 days, then after 10 days and so on. Anything to make the battery last longer. The electronics to do this would be simple enough and would consume only very minimal power.
The people that wrote the specs probably never considered a situation like this - which is hardly surprising!
And why not a second ULB with a far longer interval at a low frequency to facilitate long range detection and rough position finding, then rely on the ultrasonic unit for precise location. It would need to have a long interval to make the battery last - low frequencies will require greater energy per pulse, but travel far greater distances. I suspect having one ULB operating at both low and ultrasonic frequencies would be rather too challenging, so on the KISS principle have two.
But that would of course not be necessary had the plane not been "lost" in the first place!