"In simple laboratory tests small nickel ingots were produced from the plant ashes. Sowing these plants on appropriate soils and harvesting them at the end of the growing season makes for an environmentally friendly way of recovering nickel. Because these plants extract nickel from the olivine lattice, for every ton of nickel in the plants 330 tons of olivine must weather, equivalent to a capture of 400 tons of CO 2 . Weathering proceeds faster under vegetation. The introduction of this method could revolutionize the nickel mining industry."
See page 8-9 of the Green Cookery Book here for more in-depth information on the technique: https://projectvesta.org/science/#dflip-df_103/9/
Or see this paper specifically on the topic: Schuiling, R.D. (2013) Farming nickel from non-ore deposits, combined with CO2 sequestration.
You're planning to put the olivine on the beaches, the nickel accumulating plants will be planted above. So presumably they'll be as exposed to the action of the sea as the olivine. The document talks of use in poor soils, but no mention of coastal or beach. You've identified species suitable for that level of salt water exposure? If the plants are beyond the high tide mark, won't most of it get into the ocean first?