ESL wrote, "Those are the rules, but I've never seen them enforced anywhere except at Paris Brest Paris. "
A few years ago, I refused to validate a handful of riders on the Snow Roads, all from the same club, following complaints from other riders that they were being supported by a "team car" on the route, including lots of single track roads.
I did include in the notes to riders beforehand that support cars could be allowed if permission was sought beforehand and there was a good reason. Nobody asked.
The best rule for supporters should simply be "Don't be a dick".