I notice the vast majority of battery discharge indication is done by use of a voltmeter. What I am wondering is if anyone has attempted or knows anything about measuring state of charge with a load testing circuit or some fancy specific gravity meter and representing the results through some sort of digital or analog readout.

the only example that comes to mind is if you want to run your stereo or pc with the engine off... read 12.8v at the battery, but as soon as the surface charge comes off, you're alot further into it than you anticipated.