For Balboa Park, where parking is usually so scarce, I'm more worried about signs, parking enforcement, and towing, to keep the spaces available for EV's. On another thread someone noted that a sign "Electric Vehicles Only" sometimes led hybrid drivers to park there in the mistaken belief that their cars were electric.
One good sign was "No Parking Any Time - Except Electric Vehicles While Charging". That would also take care of the problem of someone charging for half an hour but parking there all day, taking advantage of the prime parking location. But that wouldn't really work, because it wouldn't allow someone to use the charging station while attending an event in the park. E.g., I'm not going to leave a performance at Old Globe Theater to move my car halfway through the first act. The simplest solution might be signs saying "No Parking Any Time - Except Electric Vehicles 3 Hours Maximum."
For a more complicated solution, the reservation system could charge something like $10 to make a reservation, waived if you actually showed up and plugged in; $1 for the first hour, $2 for the second and third hours, and $20 for each subsequent hour.