Anyone can do this: just carry a washable felt pen or a board marker with you, and you can grafitti any smooth (glass/plastic) surface in a non-permanent way.
Grafitti needn't be permanent, this way we can bring web-style comment systems to the solid world in a simple, non-destructive way.
Don't
like what you see? Rub it off with a hand or a bit of fabric.
Like the Roman fora, but brought up to date.