Would not the 777 have ADS-B? So ALT reporting would be inhibited but ADS-B would enter GND mode?
IIRC, YSSY, YMML and several others with newer ATC systems have the ability to electronically squelch ALT returns, yet utilise the ADS-B return for ground movements.
The US FAA says as much in their AIM:
. Aircraft equipped with ADS−B (1090 ES or UAT) must operate the equipment in the transmit mode (on position) at all times while on any airport surface.
NOTE− Pilots of aircraft equipped with ADS−B should refer to AIM, Automatic Dependant Surveillance − Broadcast Services, Paragraph 4−5−7, for a complete description of operating limitations and procedures.
And from the Australian AIP ENR 1.6 7.1.9:
A pilot operating a Mode S transponder must:
a. Enter the aircraft’s identification that corresponds exactly to the Aircraft Identification shown in Item 7 of the flight
notification filed with ATC for the flight; for those aircraft that are capable of reporting Aircraft Identification. The ICAO
defined format for entry of the Aircraft Identification shall be used except for domestic operations when VH is not to be
entered on the flight notification. (e.g. VOZ123D, REX638, QFA842, VHQFO (international), FDA...)
b. On receipt of ATC clearance, or requesting the earlier of Push Back or Taxi, select TA/RA/XPDR/ON AUTO as applicable.