NOTE: This recipe is purely theoretical, a complete joke, and is not intended to be carried out. Ever.
Thanks to Arielle's Recipe Archive (recipes.alastra.com; "Pit Roasted Pig") and Bob Arson/Church of Euthanasia ("Butchering the Human Carcass") for reference; the rest is my twisted imagination.
First, if it has not already been dressed, prepare the carcass: Suspend the body by either pairing and tying limbs with rope or twine and attaching these to a horizontal beam, or by inserting meathooks into each ankle via a cut behind the Achilles Tendon.
Completely bleed carcass by making a deep incision across the neck from one ear to the other. Be sure to raise the rest of the body above the point of incision so that gravity is allowed to pull out all the blood; this may be aided by repeated compression of the abdomen. Approximately six liters of blood should be removed through this method; discard carefully.
Remove head by continuing already started incision around the entire neck and twisting and/or slicing to separate the head from the spinal cord.
Gut corpse by cutting from the solar plexus to a point near the anus, carefully avoiding the intestines. Cut around the anus and tie it off with twine. Using a saw, cut through the pubic bone. It should now be possible to remove the anus and all abdominal organs by pulling and cutting as necessary. Take care not to cut into organs of the digestive system, as they may contaminate the flesh (clean thoroughly if this happens). Cut through the diaphragm to reveal and remove chest organs and any remaining blood vessels.
Remove feet and lower portion of arms by cleanly cutting around and separating the ankle and elbow joints. Removing all hair at this point is suggested. (enth notes that pork factories do this with flash heating; if a little surface charring is tolerable (roasting it in the pit will char it anyway), try taking a propane torch to the skin, or employ some similar method of burning off the surface hair.) Saw backbone in order to lay body spread eagle while roasting.
Prepare fire pit by digging a hole in solid ground approximately 1 to 1.5 feet deep, 3.5 feet wide and 6 feet long, tapering out at either end. Fill the pit with hickory twigs and burn until dry. Form, light, and maintain a second fire near the pit for use as a source of coals during the cooking process.
Combine water, salt, pepper, and vinegar to form a brine. Suspend body, spread-eagle and meat side down, over pit using iron rods, bed springs, or a heavy hog wire mesh. Roast very slowly over a period of 10-16 hours, or until the internal temperature of the meat reaches 170 degrees Farenheit. Once cooking has begun, cover with metal roofing sheet or other heat-tolerant material in order to retain heat and distribute smoke. While roasting, occasionally baste meat with the brine and add coals as needed. Coals should be limited so that grease does not drip and ignite; these flames should be smothered with a shovel to prevent charing or burning. During the last 5-8 hours, turn the body over, skin side down, and collect fat that may accumulate around the ribs and shoulders. This can be solidified and stored as lard for later use.
Once the meat has reached 170 degrees, remove from pit (or remove coals) and smother with barbecue sauce. Proceed to chop off pieces of meat (1/2 to 1 pound per serving) and enjoy! There should be enough meat to feed a small to moderately sized group of people; approximately 12-24 servings.