The only small things I can think of are annuals (alyssum, vinca and/or impatiens,
Nigella hispanica 'African Bride' (which will self sow)),
Cerastium tomentosum,
Nemesia, or
Dianthus (which has the added benefit of grey foliage).
I don't know if you would consider them small or not, but I use a white flowered
Pelargonium throughout the Moon Garden (both the floral and leaf shape are a nice contrast to the daylilies), and I also have a few white flowered society garlic plants (which you need to search out online; I got mine from
http://www.secretgardengrowers... ).
I did have two different white flowered daffodils in the Moon Garden, but either irrigation issues or last year's terrorist gopher did for one clump. I had/have 'Misty Glen' and 'Stainless'; both are nice, and if you can work it so that you can plant them behind something dormant that will come up later and hide the dying foliage, that would be great. (Another great white daffodil is 'Thalia', but I already have a large planting of that under a willow tree, so I wanted something different for the Moon Garden. 'Ice Follies' is good too, but it is not pure white; the cup starts out yellow and then fades to white.)
