My registry wouldn't be of much use if I signed bogus blocks, because people would just unsubscribe from it, and because the other 11 (or n) operators probably aren't going to be signing the same bogus blocks, any manipulation I make would likely get outvoted by the others. The network shouldn't ever grow "dependent" on my service - it need only have a spot where the user can pop in a URL to subscribe to block validity hints - the same way you can point a mail server at the antispam blacklist of your choice since they all have a similar DNS-based protocol. My service would essentially be to offer a tiebreaker to choose the more legitimate of two competing blocks, each hint I sign would be open to scrutiny. If I start publishing crap (for example, I am favoring a revision of a block that contains an obvious double spend against an earlier revision of that same block, or condemning a block with perfectly valid transactions, or am attempting to roll back several blocks at the same time without a flamingly obvious well-known good reason), people would see this, they would ignore me and go elsewhere for the same service.
The damage would already be done. The double spend would have already occurred. The theft/fraud completed and irreversable. The fact that in future people would stop using the service would be immaterial.
While you may think 11 other providers would form they may not. Mt. Gox still controls 90% of currency trades. The top 3 pools control 70%+ of mining traffic.
I understand the concept you are describing however it relies on you always being factual and accurate. Through hacking, sabotage, coercion, or bribery you may not be so any security it provides is a false sense of security. In other words it will work until the moment it is needed the most and then it will fail.