Not too sure where you got your info from On-MarkBob,
There are plenty of a/c which use the airframe to earth both a.c and d.c. Including the B767 that you operate. The important thing is that the earths are segregated to prevent cross circuit interference. This is not just true of a.c to d.c interference but also different phases of a.c too.
In the nose of the 75 and 76 are a battery, battery charger and static inverter. The earths of which are all within about 3 ft of each other! (all the earths are to the airframe)