Just in case it should be of any interest, quite a bit of work has been done with deep substrates in reef tanks over the past decade or so. H2S build up can be largely avoided by having a large and healthy population of benthic microfauna (the usual suspects; polychaetes, copepods, amphipods, peanut and sphagetti worms, etc)within the substrate. Such organisms are commercially available from various aquaculture operations (Indo Pacific Sea Farms, Inland Aquatics, etc). If you're curious about deep substrate options in a saltwater aquarium, I'd recommend Shimek's old Aquarium Frontiers articles (ca. 1996-1998 or so) on the subject as a starting point. His bibliographies list a number of quite useful papers on marine sediments. Adey addresses the subject briefly in 'Dynamic Aquaria' (Smithsonion Press) as well, if I recall. There is also a very simplistic overview summary of deep sand beds in reef aquaria at http://everything2.com/index.pl?node_id=1278760 which is a bit outdated, but still fairly informative.Seawater has over 200 times mores sulfates than most freshwaters. That fact creates one major problem-- H2S or hydrogen sulfide toxicity. Sulfates are converted to toxic H2S in anaerobic environments, particularly a soil underlayer. The combination of a high sulfate concentration and an anaerobic environment is deadly to both plants and fish (my book, pp 67 and 133). In contrast, the typical freshwater NPT can have a soil layer with a strong anaerobic environment without problem, because there's not enough sulfates in freshwater to get converted to H2S.
From a practical standpoint mangrove seedlings just grow too slowly to have an appreciable effect on water parameters. Julian Sprung experimented with them in a reef tank back in '94, and Albert Thiel did so quite extensively back around '97 if I recall correctly. For nutrient export something like chaetomorpha (on the algae side of things) or even xenia (on the coral side of things) will be tremendously more efficient than mangrove seedlings will.That said, using mangroves in a sump tank connected to a main seawater tank sounds good. The mangroves, which have the "aerial advantage" (my book, Chapter IX) would efficiently remove waste and excessive nutrients from the main tank.
FWIW, I agree with the "unethical" part. It's why after twenty five years of involvement with the saltwater hobby and industry I'm back to keeping freshwater tanks againMarine tanks are tricky, labor-intensive, expensive, and in my opinion, unethical.