Whisk the flours, salt, and yeast together in a large bowl to combine. Add the water and stir with a heavy spoon until a sticky, shaggy dough forms; it doesn’t have to be completely smooth. Cover the bowl with plastic wrap and let it sit in a warm spot for anywhere between 12 and 20 hours.
The dough will be wet and bubbly. Transfer it to a lightly floured work surface; sprinkle the top with a little more flour and fold the dough over on itself once or twice. Use a metal bench scraper, plastic pastry scraper, or spatula to keep it from sticking to the surface as you fold. Cover it loosely with plastic and let rest about 15 minutes.
Generously coat a cotton dish towel (not terry cloth) with a layer of cornmeal. Using just enough flour to keep the dough from sticking to the work surface and your fingers, gently and quickly gather the dough into a loose ball and transfer it to the prepared towel; dust the top with more cornmeal. Cover with another clean towel and let the dough rise until it more than doubles in size and will hold the indentation for a moment when you poke it with your finger, 1 1/2 to 2 hours.
Put 2 tablespoons of the oil in a small bowl with the rosemary leaves and a sprinkle of salt and pepper. Stir with a small spoon, using the back to muddle the leaves.
Start the coals or heat a gas grill for medium- high indirect cooking; 450°F with the grill closed is ideal. Put a 10- inch ovenproof skillet in the grill over the indirect side of the grill; close the lid. Bring the dough, still in the towels, out to the grill, along with the grapes and oil mixture.
When the grill is ready and the skillet is scorching hot, after 10 to 15 minutes, carefully remove the skillet and close the lid again. Pour in the remaining 2 tablespoons oil and swirl it to coat. Use the towel to transfer the dough to the skillet. Scatter the grapes on top and drizzle with the rosemary– olive oil mixture. Shake the pan a few times to distribute the dough if necessary, return it to the indirect side of the grill, and close the lid. Bake, undisturbed, until the focaccia almost fills and separates from the pan and is golden on the top and edges, 30 to 40 minutes. (The internal temperature should register between 205° and 210°F on an instant-read thermometer.) Transfer the pan to a wire rack and let the focaccia cool in the pan until you can touch the bottom. Cut into slices and serve.