You can either replace the whole site with one of your own creations, or simply tag the site with a moniker of your choosing. The electronic signs in the New York subway are not connected to train switching systems. The worst thing a hacker could do would be to change the message on the sign. And because the ramifications of breaking into an electronic sign are so utterly trivial, the security mechanisms are all programmed at the factory to respond to the same password. In short, changing the message on the sign requires as much technical skill as spray painting graffiti on a subway, and is just about as dangerous to the public.
Culture jammers depend on free access to the Net.