Alot of boost guages ARE mechanical... that is the tool you need. They are a simple vaccum guage.
obviously you have never owned a performance auto car as alot of guys who do drive performace autos go through their gears manually and not just simply place it in drive. Thus having a shiftlight wouldnt be entirely useless especially at the drags. Although i dont really see the point of this in a car only used on the street. Lets face who needs to know their optimum shift point when going down the road to get milk and bread.