Ok, my mind is going again, and I've another idea.

<Long winded pre-explaination>
Seeing something online about iron gall ink today reminded me that "invisible" ink could be made from iron gall (ferrogallate) ink. If one component of the ink is written onto the page, very little in terms of writing will apprear, however, if the other component is washed over, the text will be revealed after a short time.
</long winded pre-explaination>

I also realized that this ink only works with Fe3+ (ferric), not Fe2+ (ferrous) iron compounds.

SO, if a light-sensitive iron compound (ferric ammonium citrate, ferric oxalate, etc., etc.) were coated onto a paper and exposed to light, therefore reducing some of the iron to ferrous, and the paper was then soaked in or coated with gallic acid / tannic acid / gallotanate, etc., would an image not be produced?

Do I have a good idea, has this been done before, or am I WAY off?