I think you may be conflating concepts. National's own insurance most certainly requires that the renter taking advantage of the coverage had permission to use it. As many of the corporations factor in the cost of insurance to the rental rate, if someone was not eligible to take advantage of the coverage, National won't be extending the benefit to that person. Furthermore, not having permission to use a given code/pricing rate technically would be a violation of the rental agreement anyways - it's a misrepresentation by the renter that they are permitted to a certain price based on an agreed-upon contract, when in fact the renter is not an intended beneficiary of that rate.

If a renter was relying on his/her own personal insurance (or credit card) for coverage, it's a more interesting question. While it still may be a violation of the rental agreement (therefore potentially disqualifying coverage, as the rental was premised upon a fraudulent misrepresentation - whether innocent or not), there may be certain statutory obligations on primary insurers to cover certain losses. I know this varies from state to state. So it could be a weird situation where the medical losses would be covered but perhaps property damage wouldn't be. It's really a fact-specific inquiry.

The easiest way to not to have to hypothesize about these edge cases though is to use codes you're entitled to use and never have to worry about it again.