Why throw salt over a barbecue before cooking?

Salt is a good means to control/suppress flames and flareups without having a cooling effect on the coals.

Possibly they were using it to slow the coals a bit. Salt doesn't really burn (though if you can get it to melt you can get a nice orange flame from the sodium burning off), and if you put enough of it on the fire at once it could smother it.

It's also possible that they believed, as @Jasie mentioned, that the salt would somehow get from the coals to the meat and season it. This would not happen. Mostly because the salt doesn't burn or atomize, so it won't rise up to hit the meat. Perhaps if it had LOTS of flavorful impurities in it, they might smoke or atomize and hit the meat--but they wouldn't add a salt taste. They'd taste like whatever they taste like.

So in the interest of assuming the cooks weren't just being superstitious, I'd go with the flame control idea: It's something that could be used to damp the fire down that would not turn to steam (ruining the dry heat you're after with charcoal), and is a safe, known compound so you wouldn't be risking getting something harmful on the meat.

I have been taught by folks using cast iron Dutch Ovens outdoors, that not only does salt spread heavily over coals reduce flair ups, it helps keep the heat in, while reducing how quickly the briquets burn. My own experience with fruit pies or fillings,is that it keeps the spill-over from burning and smoking for prolonged periods. Sure works in my oven for making spills easy to clean up and eliminating fire.

I don't think this provides a full answer to your question, but the following snippet I came across might at least provide some clue:

Solid salt crystals melt at 800°C and evaporate at around 1.500°C, temperatures reached in wood fires and glowing coals, which can vaporize salt and deposit a thin film on foods above them.

(Source: On Food and Cooking, Harold McGee, p. 643)

But this appears in a brief section on "The Physical Properties of Salt" and there's no further explanation as to the implication on cooking in practice, that is, whether the glowing coals in an average barbecue get hot enough to vaporize salt and whether the film that's deposited on the foods is enough to make a noticeable difference in the taste of the food.

Though it's been years since the original question, I just recently found another possible answer: Apparently when cooking with Japanese Binchotan charcoal, chefs will throw rock salt at the coals to knock off the white ash around them, exposing more of the red hot surface.

Of course, binchotan is really a whole different category of charcoal from what most people use--it gets to something like 1500 degrees F.--so it may be that the salt would have a different purpose than on "normal" charcoal.

In the case of burning frankincense you would put salt on the charcoal so you don't get the smoke and unpleasant smell and only smell the herb or root your burning. It also controls the speed at which frankincense burns. I would think that may be a valid reason at a barbecue.