Dark Wallet is probably the closest to actively implementing something along these lines in a client.

See:

https://wiki.unsystem.net/index.php/DarkWallet/Alpha#Mixing

When sending, the transaction is sent to an internal mixer, that starts announcing on the coinjoin channel, if another peer answers, both will negotiate a shared transaction that includes both peers inputs and outputs. If successful the final transaction will be sent into the network.

You can see how a transaction from the mixer looks https://blockchain.info/tx/c38aac9910f327700e0f199972eed8ea7c6b1920e965f9cb48a92973e7325046

And also the multisig section that follows:

Multisig funds are special bitcoin addresses that can be spent from only by a number of participants signing the transactions. This means accounts can be held among several people or devices, and several signatures (above some threshold) will be needed to spend.