I put mostly old used dry rock and sand in a bucket with tank water and a powerhead. After two weeks it stank. after three it didn't. after a month I did a water change with old tank water.
After two months and the first water change with clean fresh water the No3 was undetectable, the Po4 was quite low. After a month in the tank with no livestock the nutrints read zero.
The sand was donated by a friend and well rinsed as was some of the rock. the other largest rock was from the coast of california a friend had fished out of the ocean and had been let dry. the last piece of rock I found In my moms backyard In Utah and quite likely came from New Mexico. Currently, 10+ mos later, the tank is fishless and the Nutrients are undetectable in a skimmer less filterless tank. I didn't clean the rock or sand btw.
Peroxide, muriatic acid, Vinegar and bleach should dissolve most of the organics, and release bound organic Phosphates from the surface of the rock. Id go bleach as a last choice pers. But the aggressive bacterial treatment while "cooking" and curing the rock will also do more than most give it credit.
Congrats Btw, 16yr old Rock is cool! I think "power washing" would be a good first step.