I demand it would be called the Claymore mod, if it were to become a minimod.
also, it's not a reliable source of clay, but Biome's o Plenty adds a biome which basically is made out of clay/hardened clay, but ofcourse you'll need to find this biome first...
I guess there isn't really a way to mass-produce clay because you don't really need it in that large quantities... the mods that use clay, only need a little bit of it, and let's be honest, there's much more aesthetic choices nowadays besides bricks...