I think you suggestion is very insightful. The problem isn't really why entities are censoring requests in transit. (Those could be rationalized till the cows come home.) The problem is that as a client, you have no way of determining if one of the internet's series of "tubes" is lying to you.
If I had to vote for one of the above suggestions "451" would be me choice. However, for orthogonality I would propose a 600 code series that would indicates you are receiving a response that originated from a proxy rather then the true destination server.
In this case 603 Ain't Gonna Get It, would seem most appropriate.