I just think that steppers don't have small enough step size to do what Fabitosh is proposing.
Nice looking experiment. Good show!
You talk about photo-transistors, but the devices in your photo look like photo-voltaic cells?
Couldn't you implement a "micro-stepping" scheme, either by using a driver chip that implements it, or by building a custom driver that can output a varying voltage to the coils (instead of the regular on/off scheme), to allow you to position the rotor between coils...?
Yes, but that would quadruple the complexity of the project and Fabitosh seems to have his hands full already.
You seem to have these connected as if they were photo-sensitive resistors. Aren't they photo-voltaic cells? They should generate some small voltage upon exposure to light. What happens if you just connect the - side directly to ground and the + side directly to an Arduino analog input pin?