It would be much easier to separate the skin and do your crackling on a rack in the oven and your meat on the BBQ, if should stay moist. I know a lot of restaurants do their crackling this way anyway, but always seems a bit of a cop out to me.
If you did want to do it on a bbq you have to start really hot, I would put it in with the lid down, don't open it for an hour and then open it up and cook for a further 3 hours for a joint that size. The key is to get your bbq super hot in that first hour.