I don't see any difference in how the 240 or 280 circuits work. The HL switch on the combo turns on power to the pair of fuses in the fuse box (L and R), which then power the common terminal of each head light through the R and RY wires. Then the dimmer switch grounds either the High or Low beam side to decide which is on at any time. The only true difference is the dedicated HL fusible link on the 280z.
The only slight year related difference is in the early 70-71 switch where the HL combo switch acts on the GROUND side of the circuit, at the center common line to the dimmer switch, while the battery + side power is hardwired to the two fuses in the fuse box. The dimmer still grounds either the HB or LB side of the head lights. Even the harness wire color codes out to the head lights stay the same across all years. I'm pretty sure the early issues were totally about the lack of connector availability.
While there other ways to approach this problem, including the one you mention to just put a relay at the HL combo switch contacts, Dave's harness that puts the relays way out at the headlight connectors, and moves the power supply directly to the battery, is the best overall solution for one very important reason.
It removes high current requirement (and the resulting voltage drops and heat buildups and failures) from ALL switch AND harness connections AND fuse box fuses areas in one fell swoop. All components (switches, fuses, connectors) are reduced to relay trigger level power levels, which even the worst condition stuff is capable of handling. The only areas the user has to improve, clean or replace might be the pin contacts at the head light connectors where the new harness splices in.