From the standpoint of phosphate leaching, there are a couple of considerations. If the rock was originally in the sea (as opposed to terrestrially-mined fossilized reef rock), then it probably doesn't have much if any phosphate in the actual rock matrix. In this case, dissolving the outer few millimeters of the rock's surface should remove any phosphate that may have precipitated during the time the rock was in an aquarium with relatively high tank water phosphate concentrations.
If the rock was terrestrially mined, then there's a chance that you might have a good bit of phosphate in the rock itself that's accumulated over the eons of groundwater penetration that the rock was exposed to. In this case, removing the outer surface may not have all that much effect on phosphate leaching potential since "new" surface is exposed that contains phosphate.
Under this last circumstance, you've a few choices that could be used to leach the phosphate out of the last few millimeters of the rock's surface. The first would be to place the rock in a trash can, and circulate heated seawater for a few weeks. That will dissolve phosphate on the rock until equilibrium is reached with the water. It would probably be enough just to change the seawater every week or so to remove the phosphate being leached and "make room for more" in the water. But you can also add an absorbent/precipitater to lock up the phosphate and encourage dissolution from the rock. Many folks do this with lanthanum chloride (SeaKleer), but you could also accomplish it by adding a fair amount of GFO in a mesh bag.
However, if you're the patient type, you could just aquascape your tank, add your seawater, run the tank without illumination for a few weeks, and include a GFO reactor. Water tests with the Hanna 713 phosphate checker will tell you whether the rock is leaching phosphate into the water, and the lack of illumination will prevent your tank from becoming an algae farm.