Interesting question. How would we know that some pilots were actually advertising for a certain supplier? Do they wear company t-shirts and hats? Do they have company stickers on their planes? Do they always win the contests? Do they autograph the products that are sold at the company's booth? Do their cars or trailers have company logos on them?

If this is being done with the intent of supporting recreational model aviation, it is arguably within the intent of Section 336. Albeit a little sleazy.