A lot of these kinds of projects render out an image and send that to the display. If you did it on a server, cut up the images, and had each kindle pull their image it should be pretty trivial. The only immediate issue that comes to mind is making sure they're relatively in sync when pulling a new image if you're worried about polling too often.
I must have 4 or 5 old Kindles in a drawer by now, somehow. I wonder if you could hack together a grid of old Kindles to make a giant e-ink screen?