I've cooked Lake trout that way before ;D
Gutted and wrapped in foil with seasonings.
Built a fire, let it burn down to bed of coals and placed a flat rock in center to hold the fish.
The fish came out excellent, a little more time consuming that placing foil wrapped fish on a grate over fire but probably less chances of burning baking it on the stone.
Another use for hot rocks is to warm your sleeping bag in cold weather.
One late SEPT trip I brought a leather pouch I made out of heavy scrap leather. Heat a nice flat rock in campfire and put in pouch.Make sure flap of pouch is long enough to fold over and wrap completely around rock. At bed time put in foot of sleeping bag and you'll stay nice and toasty for hours ;D