I live surrounded by farmland and let me tell you that the average farmer doesn't give a stuff about spray drift or advisory this, that or the other. Regulations are seen as having been drawn up by ignorant bureaucrats who "don't understand country ways." In thirty years here as an ex-townie with a bit of biological science background I've seen it all. Farmers are generally a long way from anyone watching or assessing. Welcome to the real world.

Oilseed rape can't be grown successfully without large amounts of chemical input.

Obviously, dust from a gathered-in crop could contain some pollen, though by that time flowering has long since passed. It will also contain lots of chemical residue and fungal spores as well as tiny bits of plant fibres and so on. All of these can cause irritation or allergies. The case for hay fever resulting from exposure to oilseed has not been made.