There are only half million Teslas so you can just distribute the VIN numbers of all of them with a single bit flag saying whether they are eligible for free charging and another bit saying that they have an account in good order. Twenty bytes per car should do it, say 10 MB of storage to hold enough information to say which cars are allowed to charge even if the network is down.

Mind you in three years of owning and driving (70k km) my Model S I have never encountered a Supercharger station that was entirely out of order, just a few heavily used ones that have one or two stalls often not working.