This is an interesting idea. Given the additive properties of bitcoin keys it would even be pretty easy to make a key that was very very unlikely to be recoverable. Three people separately generate keys, throw away the private keys, and then mine to the product of the three public keys. It would take all three of them lying about throwing away the private keys and then collaborating to recover the combined private key to get the coins back.
Another possibility is that one (or more) of the 3 installs spyware on the other collaborators before the generation (or at least before the subsequent destruction) of the private keys. Then the attacker waits a while and then combines all the captured keys to produce the combined private key and claim/spend the mother lode.
