Another alternative is to simply use jquery like framework to create a modal dialog window and display the message. Hide all the close buttons. You can put this script in the wordpress header and then all pages will show the dialog when the document is done loading. This has the least impact on your domain and content wise.
I wonder if a JS redirect would work too to some central page that any number of people can use (maybe a 302 redirect would be better.) It can display the desired page in an iFrame. I don't know what could be on that central page, but it could get a ton of traffic if done right. Donations, petitions, forum, guest book, chat, politicians contact details.. etc. Perhaps it can be displayed as the requested url somehow.. is that possible?
The other thing it could do is redirect back to the source site every x visitors, so one could set the ratio: 1/3 visitors gets the redirect. Search bots don't get the redirect.
It'd also make a good point for media coverage, providing some metrics on the effect of the blackout.. so long as people use it.
"xx,xxx internets were censored in the last yy seconds/minutes/hours from zz domains"
