Many folks tried add on gauge pointer type devices, most optical. you looked for a change in reflected light from a presumably shiny pointer. again results are inconsistant. Getting the mechanical mounting system built to the gauge is the hard part. The electronics could be pretty simple with just an infrared LED and phototransistor feeding something not much more sophisticated than a light probe.
You can try this is you have an "Active' light probe by moving it over the gauge's surface and see if you get a sharp change in pitch near the pointer. Often reflections from the gauge's surface mess you up. Tom
