Did you read about a concept of distributed oracles?

https://github.com/orisi/wiki/wiki/Orisi-White-Paper

and the source code repo:

http://github.com/orisi/orisi

While it's quite different from what you're proposing, distributed oracles also allow for handling banking/trading offchain, using multisig accounts. It would definitely be worth for you to see what's being done in this area