Capturing signals and reconstructing them isn't hard when you fly special mission aircraft.

But you can't just strap a picocell to a SAR aircraft and expect it to work.

Also cellphones don't have that much of a battery life if a hicker has one its most likely going to be either off due to them being out of coverage or dead due to battery drain by the time anyone figures out they are missing.

Considering the cost of such platform and the little to no use it will bring the money is probably better invested in high resolution FLIR cameras and more aircraft since anything that could ping a phone and get its location would cost more than a civilian SAR aircraft.