Rinse salmon and pat dry. Mix rock salt, brown sugar, and white pepper. Spread half of the mixture over bottom of a 12- by 17-inch pan lined with plastic wrap. Lay fish, skin down, on salt mixture. Pat remaining mixture over salmon. Cover and chill 2 to 4 hours. Lift fish from pan, rinse thoroughly, and pat dry.
Meanwhile, select a site that is protected from the wind. Build a fire in a portable barbecue (20 to 22 in. wide) with a firegrate, vents open: About 2 1/2 hours before serving time, ignite four or five seasoned, split logs (each 4 to 5 in. wide, 12 to 14 in. long) on firegrate. Let wood burn down to medium glowing coals, 1 to 1 1/2 hours; a few low flames are fine. Judge heat by holding your hand where fish will be. When you can barely hold your hand in this spot for 5 to 6 seconds, the fire's ready for cooking.
Load salmon onto soaked frame.
When fire is ready, set frame with salmon at a 45° to 60° angle over the fire with flesh toward the fire and wide end of fish 1 1/2 to 2 feet from heat, sticking stake ends into a hole to hold it (or lean frame against the barbecue). Use rocks, concrete building blocks, bricks or bagged sand to brace frame base securely. Check temperature by placing the back of your hand against the fish at the top and the bottom; you should be able to hold your hand in place for only 5 to 6 seconds. Adjust by pushing coals away from fish if too hot, closer if too cool.
(To use a gas grill, turn heat to high, tip lid open, position frame over heat, and use your hand to judge cooking temperature. Move fish closer for more heat; turn down gas for less.)
Mix butter with lemon juice. Baste fish several times with butter mixture as it cooks. Check heat often. If wood frame starts to smolder, squirt or brush with water.
Cook fish until surface turns evenly opaque, 20 to 30 minutes.
Handling frame gently (cooked fish breaks up easily), rotate salmon so skin side faces the heat. Secure frame and continue to cook just until fish feels firm to touch, 20 to 30 minutes more, basting several times.
Gently lay salmon in frame, skin down, on a large board or platter. Snip wires and gently pull wood frame from fish. Serve salmon hot or cool. Lift fish pieces off the skin and season with juice from lemon wedges.