> I like the idea of an area of LEDs. I wonder what kind of resolution / > LEDs/inch one could get.
Standard LED laser printers are up to 1200 dpi (8.5∗1200=10,200 LEDs), no reason why you couldn't do 1000 dpi. You'd only need one row of them, so for a 12" wide machine you'd need a strip of 12,000.
The trick would be engineering an emitter that could do UV with sufficient energy to expose the film as it passed by.