i have seagrass in my fuge but not for nutrient export, rather to create another ecosystem niche in my tank. experiences i have made with it:
needs lots of flow and does not like the amount of solids that settle down there. both are conditions usually existant in most fuges, so i would not recommend it.
it is a slow grower compared to my caulerpa prolifera, and by far not as easy to harvest if even possible, that makes it no good plant for nutrient export.
if you like seagrass, try it for its looks and interesting ecosystem. you need mature sand to put it in, sand height depends on the species. there are species, usually with thin long/small round leaves, that cope with thin layers and moving sandbeds, even in reeftanks. large bladed leaves usually need deep and steady substrate.
i have halodule pinifolia and uninervis both in the display and fuge, the thinner one grows under reef conditions even better than in the fuge. for nutrient export i use caulerpa prolifera on rocks in the fuge and vodka dosing.
greetings martin