I think it would really help both in terms of outcomes as well as the perception of police if they gave this kind of clear description of what's going to happen. As it stands now, the curfews aren't enforced with any kind of regularity, so they just cause confusion.
Imagine this scenario: Maybe 80% of the protesters are totally planning on going home at 8pm. You have 15% who are stubborn and don't like to be told what to do; they'll go home at 9pm if they're asked to insistently, just to show that you're not their boss. And 5% who are downright looking for a confrontation and won't go home until it's clear they won't get one.
What happens when you apply your rubric?
Well, when you apply your force at 8:30, those 15% move from the "stubborn" camp to the "confrontational" camp. In the 80%, there will be people who see the unnecessary violence, and move to the "stubborn" camp; in addition to people who tried to get home by 8:30 but couldn't for whatever reason.
Now you've got 30% of your protesters in the "confrontational" camp. The police get more defensive, and start doing stupid things like shooting people before 7:30. Congrats, now 40% of the people are in the confrontational camp.
Malcolm Gladwell recently posted a chapter from a book he wrote, concerning The Troubles in Northern Ireland, as a podcast recently; it addresses one of the core assumptions in your suggestion, that people are simple cost-benefit calculators, and so that with enough force, you can make impose your will on people. It never really turns out the way people think it will.