Just like a dog walking on its hind legs one marvels, not that it is done well, but that it is done at all.
No-one has yet said "it can't be done" for all values of "it", but LEDs are not designed to convert photons to current, so there is no particular attempt in the manufacturing process to make them so.You're going to need a fair amount of amplification to do this, and that is going to be tricky and expensive in a large rectangular array, so the idea of making a linear array and scanning it either my moving the array, or by moving the image over it has a lot going for it.
Grumpy_Mike has some information on using LEDs as sensors - http://www.thebox.myzen.co.uk/Workshop/LED_Sensing.html
Instead of trying to read 900 sensors at once why not read one sensor 900 times
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