It could work. I've made some money by doing henna on people in a hostel. I sat down in the common room and started drawing on my hand and people came up to me left and right asking about it, so I offered to give them something small for cheap or free. Inevitably they'd want a bigger design, lol. It was a slow time too, I'm sure I could have made a frickin' KILLING in the summer (if only I could bring myself to go to Europe in the summer).
A friend of mine got stuck in Amsterdam and needed about 50 euros to pay for an unknown departure tax on her airline ticket. She had 3 euros to her name, so she sat in a pretty, crowded park and started drawing on herself. In a few hours she made the 50 euros and then some.