> I have a partially complete refactoring of the camera code that should
> speed things up; in the meantime, I wonder if a simple resistive light
> sensor run through the mic input wouldn't do just as well.
> 

A cadmium sulphide photoresistor (ORP12 equivalent) worked OK measuring resistance in Turtle art

Cover cell with little finger and hold near bright light. Pulse gives +-50 ohms in 3000 ohms.

Screenshot attached