"Resistance is futile"
The Borg of Star Trek were wrong, resistance is not futile, at least when it comes to fixing the pre-amp in this guitar.
Problem: the G string is much louder than the other strings.
Solution: more resistance.
The pickup has separate piezo transducers for each string, with six trim pots on the pre-amp board to adjust volume balance from string to string. Unfortunately, this doesn't work as advertised, and appears to be (as some guitar forum posts would suggest) a common problem with this model. The output of the G string with its trim pot turned all the way down is louder than any other string turned up to max.
I managed to locate a schematic of the pre-amp circuit board (saving me the hassle of tracing it out by hand). The area of interest is the mixing stage where the trim pots are located...
Each transducer is connected to a 5 M trim pot in series with a 4.7 M fixed resistor, an adjustment range of 4.7 to 9.7 megohms. That's not sufficient for my honking G string, but another resistor in series should tame it. Since the trim pot is 5 M, I need to add at least 5 M to shift the adjustment range to something useable. Luckily, I had a 5.1 M resistor in my parts drawer.
The 7-wires from the pickup (one for each string plus ground) are soldered to pins on the pre-amp circuit board. I unsoldered the G-string wire (3rd pin from the right) and soldered the resistor in its place, reconnected the wire to the other end of the resistor, put on some new strings, tuned it up, plugged it in, and easily adjusted the G-string output to balance the others. So simple, I can't believe I didn't try this fix years ago!
The amplified tone is remarkably better than before, but I can't say for sure if it's the new ebony saddle shim, or just the fresh set of strings. It will take a couple of days for the nylon strings to stretch enough so I can record some audio clips for reference. That will help in evaluating whether there's a significant improvement when I try the traditional bone saddle with a modern transducer.