I think it would work but I don't go for metal stands. Since you are in Florida, you don't have the earthquake problem, so probably much less risk for you.
For your 150, just get a good modded Maxi-Jet 1200 down in the bottom to keep the detritus in suspension. Move it maybe once a week to direct flow to a new area, and that would probably help a lot. That issue is why I don't use rock in a fuge.
and just have the little flood lights shooting up toward the leaves...
I don't see this working out because the plants will grow toward the light. You would end up with some wacked Mangroves. BTW, do you already have the plants? I have quit a few really nice ones I need to get rid of.
As far as replacing a nitrate reactor with a "natural" solution, I am not sure that's an apples-to-apples deal. Personally I would go that route because I have somewhat of a fear of nitrate reactors due to the sulfur and crash stories I have read, but I would assume that a properly maintained reactor would provide better nutrient reduction than some of the natural methods.
For instance, refugiums really (IMO) do not do much for nutrient removal unless they are sized extremely large. I like them as a predator-free zone and a point of interest, but find that mine had annual cycles of sort of boom and bust.
I love mangroves but I doubt they have much ability to absorb nutrients as well. Plus they grow extremely tall and most systems cannot handle that height. I mean, they ARE trees.