An intriguing project!
On the "linear vs elliptical" issue: it's been recommended to keep it linear, but only because of the hand process (i believe). The important question is: how are you going to keep it flat?
You've got that granite slab as a start, but how on earth are you going to maintain even flatness? To me, the elliptical solution sounds better, but I'm not sure, nor convinced, that you'll be able to design a block clamp that'll remain parallel to the granite's surface.
As for holding the sandpaper down, you should definitely avoid using some kind of adhesive, between the sandpaper and the block, because that would make the surface uneven. You probably want to secure the sandpaper at its edges. The problem, again, is that we don't know how flat the sandpaper itself can be...
You've got another problem to resolve: how are you going to measure your results?
Don't get me wrong, I like your idea. I think you could actually market this as a service, if you can get right. It's going to depend on you doing one thing: identifying the sources of error/deviation, and solving them.
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