What an awesome project! May I suggest planting white wave petunias directly in the ground. They will sprawl and peak out in between the other plants. Dreams petunias are a small plant, big flowers and if planted in 3's will make a nice clump.
Some unusuals that will give a little later white flower are Venidium (Zulu Prince- easy from seed), It will close at night/cloudy days so maybe not the best choice for a Moon Garden
Abyssinian Glad (Acidantherea) aka Peacock Glad, which I like to plant in a clump of 10 corms. Very fragrant and Costco sells a huge bag in the spring for excellent $$
And if you have enough sun, a giant white dinnerplate Dahlia
