Great idea. I've heard of people putting an orange and black balloons in the mailbox with a letter and asking people to blow the balloons up and attach them outside if they are ok with trick or treaters coming around.
We're going to do something similar this year. I love Halloween!! I spent a year in the US many years ago and Halloween was just fantastic. The streets were filled with children dressed up in costume and jack-o-lanterns outside houses. It was one of the best nights I had there.