I just had a slightly different idea to help alleviate this problem.
What if each "back-in" type stall had a parking (oops, I mean 'charging') spot on both sides so two cars could back into one stall ('back-to-back' style).
If one car is connected and charging for too long or reaches 90% charge and the owner isn't anywhere to be found, the charging handle automatically unlocks, and there's an orange ring to show other people "hey, this car is now idling, feel free to take the charging cable". Or there's a phone number to call Tesla, get an agent on the phone with the stall number, and they'll unlock the charge port for you and then commence parking fees on the car that's just sitting there.
Yeah, other than requiring tesla to pay/lease/setup two spots for each stall, the rest is software.
Oh, and if you absolutely needed a 100% charge for some reason, there should be a way to 'opt-in' to a range charge, but Tesla would have to make sure that isn't abused.