That's their natural environment, isn't it?
http://discovermagazine.com/2011/apr/21-mangrove-tree-captures-carbon-filters-saltwater-stops-storms
I have mine rooted in a caged chaeto ball in my quarantine tank. It's really more cosmetic and I like having another nutrient sink that doesn't take up any room in the water.
Also, unlike chaeto or hair algae scrubbers, they're immune to overgrowth by dinos or cyano... you're right that they're not the best scrubbers...
My favorite is this plant: Salicornia europaea
Here are some good articles on options
https://www.researchgate.net/public..._conditions_typical_of_aquaculture_wastewater
http://www.ncbi.nlm.nih.gov/pubmed/15921271
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4109415/
http://www.ncbi.nlm.nih.gov/pubmed/22818948
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4052567/
http://home.eng.iastate.edu/~tge/ce421-521/ishadeep.pdf
and this article
http://www.mdpi.com/2071-1050/6/2/836/pdf
try googling this and see what you learn: "nutrient uptake Halophytes sewage wastewater biomass"
Halopytes are plants that grow in saltwater.
I haven't found anyone else who's taking this seriously in the hobby, but sewage management is an excellent resource for learning how to process water.