I wouldn't worry too much about other cars plugged into the circuit. The only time other cars will plug in for the block heater will be in winter. In winter there is no point to trying to charge a Model S on 120 V because all the energy gets diverted to heating the battery rather than charging. In other seasons, might be helpful to have a trickle charge to add a few miles overnight.
(Whether it is worth plugging in to keep the battery somewhat warm on a very cold night is a different question.)