I hope that doesn't mean I plug in my car at 80%, hoping to top it off for a long trip, but since everyone else is averaging 60% and there's peak demand, it decides to drain the battery and generously put a dollar in my account.

It would most likely be opt-in, as a form of this is already a thing in some place - you can choose to have the power company reduce the power of your cooling/heating during peaks in demand in return for a discount on your power bill.