I think instead of reservations, it would be better to notate an "intent to use" at a certain time/day. Several options would pop up here:
1. If the driver gets there before the intended time, the future "intent to use" leaves the system.
2. Since the Model S keyfob has an NFC chip inside, it could be used for some kind of mini kiosk that says "I'm here". Just wave it there for a quick "check in" and you wave over it again for a "check out". Also, the checked in (or occupied) stall use would automatically expire your "intent to use" after 60-75 minutes if you didn't manually check out.
3. All of this could be sync'ed and integrated into the Tesla smartphone app. When another driver pulls up with intent to charge and sees all stalls are occupied, if the cars sitting there are at 80% and over, their phone gets a notification that someone wants a charge. The driver waiting for the charge would use the Tesla app to send a request to the drivers that are currently charging. They can choose to ignore it if they want to either top off, or just don't care.
I'm sure Tesla is looking at something like this, but location based charge notifications would also be good. As a Model S approaches a Supercharger, it will go online and see if the stalls are all in use. If they are, a UI prompt asks the driver if they intend to charge. If they choose "yes", the same requested intent to charge is sent to the smartphones of the drivers that are occupying the stalls.
Maybe it will make the world a little bit more polite, too