I'd have thought that it would be just as easy to connect the optical sensor, since the output circuit is much the same as a hall d=sensor, effectively open collector. However, I've built a Hall sensor for my big mill for a tachometer, it uses a tiny neodymium magnet about 4 x 2 mm araldited to the spindle pulley, and the sensor "sees" it from about 5 mm away - so you dpon't need a big heavy magnet. And just stick an equivalent weight bit of steel diametrically opposite to balance if it's a concern.