Ok, equant, here's an interim method that might end up being the final method. It doesn't absolutely minimize power, but it provides a large reduction with almost no effort.
The idea is to disconnect L1 (it's the toroidal inductor located at the top edge of the analog board) from C2 (a 4700uF electrolytic; it's also on the top edge of the analog board). Use an SPST switch to restore that connection reversibly. When closed, the Mac works normally. When open, it shuts off power to the CRT filament, horizontal deflection, video and HV circuits. Those are significant power consumers, so disabling those sub-blocks will dramatically reduce the load on the power supply.
About the only unneeded block not disabled is the vertical deflection circuitry. The effort required to shut that off is probably more trouble than it's worth.
Finally, it's a good idea to add a diode across L1. Something like a 1N400X (where X can be any value). Connect it so that the cathode (marked with a band) is connected to the side of L1 that you didn't touch. The diode may not be necessary, but its presence will assure that no sparking will occur when you open the switch, prolonging the life of the latter.
I hope that this verbal description is clear enough to get you going.
IMPORTANT DISCLAIMER: I have not actually tried this procedure, so I can't guarantee that there isn't a hidden gotcha. So proceed with that it mind.