In article , says...
We burned the phosphor mid-CRT pretty badly, but the TV was a
throwaway anyway, so it didn't matter.

You could detect the incoming signals and feed them to the
CRT board to modulate beam intensity.

BTW, I'm not convinced a regular stereo amp is suitable to drive
a yoke.

At least not without some extra work. Yoke windings are
predominately inductive and intended to be current driven
(implying high impedance source), while stero amps are
designed to be low impedance voltage sources driving mostly
resistive loads. Add to that the fact that the two windings
are usually optimized for different signal voltage levels
(12~40 Volts vertical and 100~300 Volts horizontal ballpark).
You could try 70 Volt PA speaker transformers with series
resistors to match/step up the amp to yoke.