Why not use floating plants and plants that can grow emersed? but the roots in your sump?
Those have an advantage, they can use the CO2 from the air, hence not limited by the CO2 available in the water. They grow faster, hence using more nitrates from the water column.
I like your idea, and thinking of doing this one day.
Edit : the false papyrus, cyperus alternifolius for example. epipremnum aureum, is also a great nitrates remover, it's a well known appartment plant, you place it over the tank, the leaves and stem outside the water, it will send roots in the water.
Last edited by micheljq; 02-15-2013 at 03:36 PM.