Phosphor-based delay

Reading about Williams tubes, I noticed a comment about phosphor-based displays:

When a dot is drawn on a cathode ray tube, the visible spot lasts for a time (called "persistence") that depends on the type of phosphor used in the tube.

I've experimented a bit using audio reverb for video signals, but what about using CRT for audio delay? Here's one way you could do it on a black and white TV:

1. Break the connections to the deflection coils on a CRT.
2. Send an audio signal for intensity modulation (the volume would have to be turned down to avoid destroying the phosphor, and the signal might have to be pitch shifted down and processed in non-realtime depending on the phosphor's frequency response).
3. Use some sort of photodiode to "grab" the light and get a signal again (the response of the photodiode will influence whether you can do realtime processing or not, as well).

A good reference for trying this out would be the various wobblevision/wave vessel hack guides available online.