Bart + Open
Bart didn't have a sign -- he couldn't be trusted not to accidentally hit someone and send the whole place up in a riot, so no signs for Bart.
Actually, he didn't do very good on the "protesting" side of things. He wasn't the greatest fan of standing around (for one) and yelling (for two) and while he knew it was important to get out there and make a difference in a completely legal way, he couldn't help but feel like he should be somewhere else, making better use of his time. Like with Wally or Jesse doing their normal crime fighting.
But Wally and Jesse were registered now. And if it took standing around and yelling to help them, then he was going to stand around and yell to the best of his ability.
Drawing his hand from one of his pockets, he looked around and tried to get the chant down, raising his hand to the sloooooooooow beat-- but he was trying! Really! He wanted change as much as anyone else here.
And then an idea struck him.
Before anyone could notice that he was gone, Bart was back with a guitar borrowed from one of the fellow protesters (what? He hadn't been using it right then) and started to play the only protest song he knew.