This is just a suggestion taken from experience in the art world...

It's super easy to project a large ad onto a large blank wall, a parked tractor trailer, a bridge overpass, etc.. a 12v battery, an inverter, a video projector, and a laptop is all you need.

A minivan with a projector sticking out of an open side door can park somewhere and project an image onto a large wall beside a busy highway.

Project a static (non-video) image - the simple "Ron Paul - President 2008" ad rather than something with motion.

Don't use the "Revolution" ad.. the more mainstream what you are projecting, the less likely it is to draw ire and get you into trouble.