I've done it in the past and even survived the redneck car chase that followed. I try not to do it now. The most effective way to get their attention is an open handed grab of the passenger side mirror as you pass the stopped car. They're forced to live with the mirror pointing the wrong way or stop and correct it.

As a pedestrian, I've also kicked in a turn signal when someone started to drive through a crosswalk while I was pushing a stroller with my 2 year old through the crosswalk.