If you simply must express your voice on a giant piece of public property, but can't get passed the illegality of defacing something you don't own, try scrubbing your design into the grime that already exists on the wall.
"Offering a refreshing and very welcome take on the contentious art form, is a group of ex Durban Vega Brand and Communications School students, who were inspired by the work of British street artist Paul Curtis (AKA “Moose”) who began pioneering his form ‘Green’ or ‘reverse Graffiti’ three years ago. Curtis (legend has it) first hit upon the idea while working as a kitchen porter in a restaurant scrubbing mountains of pots and pans. One dreary evening while trying to erase a grease stain on the sink wall before him, he discovered he had cleaned a large white patch onto the grimy surface. It didn’t take long before the aspirant street artist began conquering the cityscapes of London, applying his vigorous selective scrubbing to more prominent walls and bridges."
This can't be considered illegal, right? It's cleaning up property. You can't be fined or arrested for that...
Let's just hope the artists know what they're doing, and keep the messages and imagery postive.