While I impatiently wait for some high speed photo diodes to be shipped to me, I discovered some more research. Someone else has created their machine, using a hexagon mirror system found in laser printer. Unfortunately the project is old and the link to their source coded has decayed. They re-purposed the mechanics on a flatbed scanner for the x-axis, using hexagon mirror for y-axis to raster the printed circuit board.
Under "Electronics" section, the person explains:
It has 12V powersupply, a speed control pin, which accepts a strange range of voltages, and a symmetrical frequency generator output (FG). This output is a zig zag shaped induction loop around the motor, which generates a feedback frequency of how fast the motor is going. This is fed to the AVR's analog comparator. The microcontroller locks this signal to a reference frequency of 2kHz, which yields 55.555555555555... rotations of the motor per second, and 333.333333...Hz scanrate."
A "symmetrical frequency generator output (FG)" goes above my head, I do not know what that is - how it works, or what to do with it. I do not know how to make a circuit to sense this type of signal. I am also a newbie when it comes to using an analog comparator - up until now I use analog input pins to sense potentiometer signal, for joysticks or interface dials.