Uh, why is this necessary? Can't you just leave a placard with something like "feel free to unplug me to use the EVSE if my green light is flashing"? 2012s can disable the charge cord disconnected alarm, and 2011s can have it disabled by the dealer if you are using public charging regularly.One of the things that was on my "smartphone app ideas" list was an app that would allow relatively blind exchange of unplug requests/grants - using QRcodes on a placard. Somewhat like the "EV-1 charger protocol" (I probably botched the source AND the name), where people who are willing to be unplugged placed a placard on their dashboard indicating so, and another placard if they weren't willing to be unplugged. People aren't always willing to allow their mobile numbers to be placed in public or exchanged with random people, hence the blind exchange.
It seems to me you only need contact if someone needs to move their car. If I were in a garage where there were more EVs than EVSE accessible parking spots, I'd do something like make it a habit to move my car at lunchtime.