OK, this is weird. I disconnected the power transformer secondaries so I could see how much voltage it delivered (one plan is to just replace the battery portion of the circuit with a regulator, and I hoped that by using the "red" primary tap instead of the "brown" I might get the secondary voltage low enough that a standard 7805-type might be usable).
Initially, I pulled the red wire from the board so that power would be applied through the brown/black primary winding only, and removed the ferrite bead so that no power would be passed along to the meter itself, in case something went awry. This way I was only checking the transformer. When I applied mains voltage through a variac, I immediately got secondary voltage with S13 (that's the front panel switch) off. Turning S13 on, the voltage went to 0. I unplugged the power cord and disconnected the transformer primaries from the board so I could map the incoming power locations to be sure I was getting AC mains in on the right pins. What I found is that the schematic doesn't match the board, if you assume the convention that the contacts represent the switch in one position; S13F is actually closed when S13 A/B are open, and vice versa.
I originally thought that perhaps the wiring was swapped around. However, the board is labeled "red", brown", and "black" on the solder mask, and the colored wires were originally soldered to them as labeled. S13 is soldered directly to the board and the wires routed to the power supply section look like original factory solder joints - though I suppose someone could have swapped them. I can't figure out why they would have wanted S13F wired backward - with the red winding connected as it is shown, this would put a higher voltage on the batteries (brown winding) when power to the meter internals is disconnected, and turning it "on" (red winding) would drop the secondary voltage just when the meter is demanding an additional 500mA of current. Of course, I had pulled the red winding so instead there was no voltage at all on the secondary in this position during my test. The rectifiers CR201/204 were open when I initially inspected the unit, so I wonder if someone didn't try an ill-conceived bodge and damage the PSU. I guess I should be glad the digital/analog circuitry wasn't fried.
I haven't checked my other 8600A opt 01 to see if it's (mis)wired similarly.