The Satellite antenna on the B777 is at the rear top of fuselage above Economy cabin. It comprises a High Gain Antenna and a separate Low Gain Antenna both have Line Amplifier/ Diplexers boxes connected. Also connected in the same area are SDU satellite data unit and BSU and other boxes are all part of the Satellite system. All in an area behind wing and well away from the Flight Deck or nose of the Aircraft. Probably for wave propagation interference reasons.
So it's fair to say the Satellite unit is autonomous to the rest of the avionics in the aircraft.
With that in mind it's probably plausible. That it could transmit handshakes by itself, as wreckage in the sea of a broken up airframe.
Any B777 techs out there, have an idea?