i dont know about peace lilies, but calla lilies would get way too large for a wabi kusa!
as states above, lobelia cardinalis would be good! this spring i got some lobelias ( likely a hybrid, not cardinalis,) and grew them in my flower garden but they were nice and compact
basically any small flowering plant that tolerates being wet..
as far as aquatic plants that bloom that how flowers that are aquatic plants that are pretty (most arent that exciting) UG has really pretty white flowers http://farm4.static.flickr.com/3640/...e8d30acd_o.jpg
bacopa lanigera has small purple flowers that are pretty, and the plant itself is very nice.