Do it more like how the service at an Apple store works: online there are multiple slots per hour, but when you get there in person, you need to check in again to get in the physical vs virtual queue.
If the online slots only cover a portion of the available charger hours, that gives a buffer for drop-ins, but also gives folks who check in online a rough guesstimate on any potential for charger contention when they plan to arrive.
So, in short, make it a very loose reservation system that only keeps track of a portion (50%?) of available charging capacity, and it doesn't guarantee anything when you arrive.
Since it would likely be far too confusing to have any sort of on-site queue management integration with the car itself, there likely can't be any "official" onsite queue management (other than driver-to-driver discussions, glaring, note-swapping, sign-in sheet in nearby store, texting, calling, etc)