I know nothing of chemistry, but I am familiar with Bonneville, having spent all Speed Week there in 1997. The salt is sticky. It sticks to itself, which means it creates no dust. Even the rooster tails off tires create no dust. The salt just falls back to the ground. But it also sticks to everything else. It is not uncommon to find a buildup of as much as 1/2 inch on any surface behind the tires. After each run, the team I was with used a garden sprayer to remove some of the salt immediately. Their pickup-truck is riddled with rust holes that you could throw cats thru.
My bus has been to BRC four times. I've made absolutely no effort to clean it, except the interior. I see no rust.
But some things will rust badly if the Playa dust gets wet, without washing the dust off. Bicycle chains seem particularly vulnerable.
If you want to wash the dust off the underside of your car, you might try what I did with my (rental) car in 1997. I put a lawn sprinkler under the car all day, moving it every hour. I could find no trace of salt after that.
And I second the motion to invent DJ-BeGone!
Elliot's Bicycle Service & Beverage Station - a 2015 BRC Theme Camp
a.k.a. Elliot's Naked Bicycle Service & Piano Bar, etc