DIY State Of Charge Meter?

I've been reading around, and found someone in the EVDL mailing list is thinking a bit like me. They posted a link recently, about a turn-of-the-previous-century SOC meter called a Sangamo.

Basically, what it does is use a shunt with a movable center tap, and the charge going in spins up an electromechanical counter to read out battery capacity in Ah, and the charge going out spins the same counter back down to null that out as it is used up in the motor. The center tap is adjusted to compensate for the fact that you'll put more into the batteries than you actually get out.

I have a few electromechanical counters around here that I could set up to count this; I think one I got many years ago at Apache Reclamation runs on 28VDC for aircraft stuff, and is a 3-digit power-on counter. That might mean that it can't be run in reverse to count back down, but if it can it'd be perfect for this.

I'm tempted to try working out how to build one just because it is old technology that is "good enough", and is certainly durable enough for the heat and vibration the bike endures. An electronic one isn't necessarily. Plus, this one would cost me nothing but time and brain power, neither of which I have in abundance but do have more of than money. ;)