I never thought about the emersed plants to help with the nutrient export. I know some of the salt water guys use mongrove trees and upon reading a bit further into it, you can use it for fresh water too. I am thinking a planted sump would be better for cichlid owners who really can't have plants in the tank due to them being destroyed by the fish.
Mangroves could work. I have emersed philodendrons (probably the fastest growing and easiest of these 3) and lucky bamboo in an open top tank that also grow with relatively low light. All 3 of these grow fairly slowly, but also don't need much light to grow. You could put a bunch of them in a sump to increase the nutrient removal.
Another plant that would work in a sump, probably better than anything, is duckweed. It grows fast and is great at removing nutrients. I know it is scary to introduce duckweed to a tank, but with surface movement in the main tank it should keep the duckweed from spreading in the main tank. I net out duckweed from my aquariums and compost it. If it ever gets out of hand you could add the goldfish from the pond sump to eat it.