Have you considered using OpenTransactions in addition to bitcoin? OpenTransactions allows an issuing party to issue certificates which can be traded. It is quite different from bitcoin but I believe it complements it well, especially in a scenario like you are attempting to construct.
It has been suggested elsewhere on this forum, but I'm not sure if you have seen it or thought about it.
I have to admit I just don't understand Open Transactions. I've read all about them, but I'm apparently too dense to see how they could do what I want (decentralized currency/commodity/stock trading). The bitcoin protocol can definitely do it, but I'm continually changing the implementation details, trying to get something easy to implement, cause I'm lazy.