Started looking into my 845AB. It is reasonably stable, but it could do better.
Bottom line: The FET opto isolator, something to tighten up the opto drive, and then removing that
1) This is an AC circuit!
The meter reading comes off of a transformer, and the chopper amplifier circuit shifts the center point of the waveform. The chopper frequency makes it all the way through to the display; the meter is averaging the output waveform. Very clever stuff that no doubt has its roots in vacuum tube designs. This architecture implies that we can't add a chopper amp in and get anything useful. That said, there is a DC segment, but it is wrapped in that AC technology.
2) That new board is simpler than it looks. It uses very good FET optoswitches to replace the LDRs and neons. It should be more consistent, because there's no dependency on the randomness or deterioration of a neon tube. You could come off of the transformer, but I think that Fluke wanted the extra control of a regulated voltage supplying the optos. The FET optoswitch is absolutely the way to go.
3) Looks like U3 (and thanks to TiN for the pictures, and Picburner for the schematic) simply provides a DC voltage to offset the zero in the front end chopper circuit. Oddly, all the other front end wiring is carefully isolated with standoffs and air wires. But this little offset circuit is just plain PCB wiring.
4) I'm not sure how important asymmetry is in the optoisolators. Mostly, I see trouble in the switching threshhold, as asymmetric switching will offset the zero. The input shaping that Fluke added should keep that to a minimum.
5) Looking at the input, if the 1M resistor is disconnected, the only DC path is through a dark LDR. Again, the opto FETs should help hereby eliminating the leakage path through the LDRs. Another point for the AC pathway in a DC instrument!
6) If anybody does get into an assist PCB for this, a small microcontroller could do a pretty good job of autozero. It would need a comparator to read the zero, and a way to slightly move the offset voltage in the front end. Could probably use pwm.