As what the others said here, BBQ and dieting are not incompatible. Smoking does not add any appreciable calories that I can tell. If you stick to the leaner meats such as chicken, turkey, and fish, you will be fine. Of course, that would depend on the regimen you choose to follow. The two things you need to watch out for is sodium (introduced mainly through rubs) which can cause water retention and sugars (sauces and glazes) that will increase the number of calories.
Chargriller with SFB, Master Forge Vertical Smoker