In my experience even a "saturated" sensor (with light bright enough to cause all pixels to read full-white, but not bright enough to cause damage) will still generate some random noise, although far less of it.
(Ironically, the cheaper the camera the more noise it is likely to have, and thus be better for this application...)
I think it is questionable whether this kind of process can generate true random numbers given a small amount of light. Tests should be conducted in worst case conditions in order to trust the system.
