This reminds me very much of the TCP Evil Bit [1]. That is, it seems like the issue of whether anybody would actually set this is essential to the question of whether or not it would work.

One could even imagine that governments would want people to live in ignorance of the existence of restrictions. It's 100% the Ethan Zuckerman Cute Cat Theory of Internet Censorship [2]. The government would rather that you have all of YouTube except the subversive content, because then you might not notice the restriction and so will learn not to care.

Indeed, from the document:

  "The use of the 451 status code implies neither the
  existence nor non-existence of the resource named in the
  request.  That is to say, it is possible that if the legal
  demands were removed, a request for the resource still
  might not succeed."

That suggests to me that the author is also aware of this problem.

It should be noted that Google has done an amazing job in trying to fix this problem by trying to force whole services to be blocked when that's feasible and by transparently explaining (in the UI itself!) when content is unavailable for legal reasons. Kudos to them.

For that reason, I would say such an effort is worthwhile even if it won't have much impact: it's quite likely that it will help further the norm that when censorship exists it should exist transparently. That's a cause worth fighting for, even if every step is going to be a huge challenge.

[1] http://www.ietf.org/rfc/rfc3514.txt

[2] http://www.ethanzuckerman.com/blog/2008/03/08/the-cute-cat-t...