China doesn't try to hide the fact that they censor a huge chunk of the Internet. In fact, they seem rather proud of it.
Perhaps they could even use 451 as an easy way to detect potential "terrorists". Sort citizens by the number of 451s they generate each month, send a cop to the top 1% of them, and publish this fact to scare the other 99%. They're already using TCP resets as a core component of their censorship regime, so I wouldn't be surprised if this Internet standard got co-opted, too.
So instead of:
You are not in the sudoers file. This incident will be reported.
They'll get:
HTTP 451. This incident will be reported.