I would use dicondra seeds or chocolate mint! if I remember tomorrow I'll show you pictures of the chocolate mint growing between my flagstones an invading my backyard grass. And it smells wonderful when you trim it. on the other hand with dicondra grass, it's an old grass that you never have to mow. It fills in the space, it almost looks like a round clover but its not a clover and you just give it a sprinkle everyday in the hot summer in all good sprinkle everyday and hot summer he should be fine.
I like how the dichondra looks, how drought tolerant is it? Would it survive a few days without watering in the hottest part of summer?