I have a customer here in VA who posts the rules and charging is done via Security and a Text message system. They had to double the amount of chargers on site to balance things out and clam things down. Everyone posted there cell numbers in the cars. it was part of the tread linked above.
I bought some stuff from Evannex and they sent me a charging placard to post on my rearview mirror. the one side basically says "hey I need to charge don't unplug my car in red. the other side says "This is an opportunity charge take the cord if you need to charge in green"
I like it and will use it. here is a link to the placard.
Charging Hang Tag / Courtesy Notice Bundle