Here is one plant that I think might work well with this idea, cat palm (Chamaedorea catractarum). To develop an ikebana layout with it I would use just one cat palm plant, and then arrange shorter riparium foliage down around the base.
Cat palm is a true marginal aquatic palm--it grows along the edges of rainforest streams in Southern Mexico--and does really well in the riparium planters.
Here's another new plant that I got--it came with the mail last night!--that might work for this idea. Buttonbush (Cephalanthus occidentalis) is swamp shrub that occurs in the Eastern US & Canada into Northern Mexico. Most temperate-zone trees and shrubs really need to be grown outside, but I found a few references to growing this plant as indoor bonsai, so it might be OK. I will find out.
Buttonbush is in the coffee family (Rubiaceae) and has attractive and fragrant white snowball flowers. I have seen wild plants in Wisconsin in backwaters along the Mississippi River and in Florida in hardwood hammock swamps.