JFYI, LiveJournal, Github and many other sites are already doing this.
Edit: Slight correction, Github responds with HTTP 200 and a "banned by your government" stub, lj.rossia.org does the same (shows the 451 code in their message though) and livejournal uses 404 code for the "banned by your government" stub. I figure it's because it's not yet in the http standard or something.
[1] https://en.wikipedia.org/wiki/HTTP_451