Yup, you could do it, but just another point of possible failure really that would only tell you in very basic terms what an ammeter tells you with more precision. Easy enough to do with coil ignition and a switch like on a car, but as a10gf says, not a great plan on a bike. The gizmo has to measure when the field is drawing more than D is sending out - ie when the battery is being drained. If you look at the things old cars have, you'll see behind the bulb holder, or in it, there's a coil etc. More trouble than it's worth I'd also say. Just keep your eye on that needle from time to time, and you won't go far wrong. If the needle flickers like a good 'un, then maybe worth getting a decent ammeter with proper damping - a lot of the modern replica ones are ****. Many of the old ones have a small adjuster screw to keep the needle under some sort of control . . .
Heresy though it be, I like the 2in Miller ammeters with a white face best. I can read them without spectacles, and they are really well damped. I trepanned the (non-original) h/lamp shell on my A to take one. No regrets, as decent electrics make one of the biggest differences between Toytown playing and Real Life riding, especially this time of European year!
