While I would support such an addition, if would not provide any additional benefits that you listed above besides better anonymity for tor users on bitcoin talk.
I'd like to suggest the creation of an .onion url for bitcointalk.org as an additional layer and a security measure against ddos attacks and (maybe) gov attacks (why not?).
It would not provide any additional benefits for security. The server would need to be the same in order for the sites to be in sync. It would really just be another link to access the same server.
Tor also does not prevent ddos attacks.
The .onion link will also serve as an backup site.
P.s. I don't know the costs related to that possible implementation.
As I said above, it would not be a backup because it is still the same server.