Saying that cooking a pig in a pit wrapped in burlap isn't BBQ is a distortion of what BBQ is. It may not be what you think is BBQ (which is fair), but it is based on the Mexican tradition of barbacoa, which is as valid a form of BBQ as anything anybody here is doing.
I've done pigs in pits a couple of times and my memory is a bit hazy. Anybody who wants the full BBQ experience should cook one at least once.
One of the most important issues doing it on a beach is to make sure that when the tide comes in, the bottom of your pit does not extend into the water table. The next issue with doing it on the beach is permission. Nothing worse than getting half-way-done, and having the cops come along and tell you to move after giving you a ticket. Both issues not applicable in your situation.
Always kept the pig below 100#'s. We typically would dig a hole, and build a charcoal/wood fire. we'd add rocks, and once the fire had burnt down to a solid bed of coals, we'd spread the rocks and coal out, and cover them with either dampened banana leaves, seaweed, lettuce or burlap. We would then set the pig on top of the covering layer. The pig would be butterflyed and in a frame or chicken wire so that we could move it around. We would then cove with the same material as above, and then place hot rocks on top. After that we would take a moistened burlap and cove the top of the hole followed by a sheet of plywood, which was then weighted down. Depending on our mood, we would either flip the pig or not. Skin came out crisp, and the meat was fall-off-the-bone. I'm probably forgetting a couple of things, but I think I have hit the important items.
Neil - Good luck and have fun.