I decided to put it behind the unused BATT slot on my dash, basically replacing that old acid-level gage with a real voltmeter. It has ground and hot leads, a very small brain (less than 1.2", longest dimension) and an LED output light. It doesn't need to be switched-hot because it turns itself off after no elec activity in 5 min. So I can check the battery voltage a couple minutes after I turn off the bike, as well as before I first hit the starter button.
These photos show how I clipped the BATT socket from the harness, ripped out the light leads (I had removed the light bulb long ago), and mangled a new hole to jam in the voltmeter's LED. It fits pretty tight in this soft light socket.
It has a color/flash scheme: the LED will be green most of the time, flashing green when it's charging right, yellow if there's a problem, and red (or flashing red) for a serious problem. These are voltage levels, reportedly extremely accurate.
The BATT lens is red, so the green light should look like dark mud. I'll drill a couple small holes in the lens around "BATT" to show a little green. And then I'll ignore it. But when it goes yellow, it'll show through as a bright orange. I should notice that. And red would come through bright red -- all behind "BATT".
I think this is way kool.

Now to put the dash back together.