Just been going over the '77 tech manual - access to the MEC would allow you to switch off all comms. Left ,right and centre VHF,L & R HF, Txpndr 1 & 2 and Satcom. Thus no ACARS transmission (uses Centre VHF) and no ADS-B (uses selected txpdr) CPDLC -if fitted - disabled due no Satcom.
You would also have access to the power centres thus gaining you entry to the flight deck. (No I'm not going to spell that out)
So you can take command, switch off the nav lights descend 500 feet and away you go.......