When I had gutters put on under my patio roof, which only has a very slight slope, the gutter guy recommended running a bead of caulk under the roofing material, right near the edge, to prevent water running back under the roof. The bead collects the water and it drips off from there. I think that's what you're asking about. I suppose a groove would work, though I think it would need to be fairly wide. But I'd go another way.
If you look under just about any granite kitchen counter, you'll see that the granite is only about 3/4" thick, and that another piece of granite, about 3/4" thick, has been glued on to the underside of the granite, flush with the front edge. This does two things: it hides where the granite meets the cabinets, and it makes the granite look like it's 1-1/2" thick. That's how I'd fashion the spill over. That 3/4" strip under your spillover would catch the water wanting to go up under, and it would make your granite look like a thicker piece. They could probably even shape that drip edge at an angle, so that the drips would fall from a point, like this (cutaway sideview):
You'd want them to do both edges, so the illusion of the thickness would be the same when viewed from pool or spa. And like a kitchen counter, you could choose the shape of the edge: square, rounded, whatever...
I'm not sure we've ever seen this done in a pool, but I can't think of why it wouldn't work...
That all said, keep in mind that on the spa side, the entire edge, above and below, is going to be underwater, so I'm not quite sure how keeping water from getting up under the granite on just the pool side is going to make much of a difference.