Perhaps the answer to that then is for the local authorities to provide basic, cheap camper van spaces at regular intervals around the route. They need to be cheap because people will be unwilling to pay - say, £25 - to park a van in a field overnight, especially if no facilities are provided.
I think banning overnight parking on the route is going way beyond what might be acceptable legally or otherwise.
My week near Shieldaig (in a rented cottage) a few years ago really taught me to avoid the NC500 completely, and there must be many others who feel the same way. A victim of its own success?