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.
Open-Transactions is basically a server, a library, and an API. (Many different services
could be built on top of it. That's why you may have trouble visualizing exactly what it does.)
For example...
An online web-wallet
could be built with it. (I'd say "like MyBitcoin" except it seems you all have recently had a "bad experience" with that service...)
An online market exchange could also be built with it. (I'd say "like MtGox", except it seems you all have recently had a "bad experience" there as well.)
Those aren't hard to visualize, are they?
Some of OT's improvements:
1) Users could utilize untraceable cash.
2) Users could use instruments such as cheques, cashier's cheques, payment plans, etc.
3) Servers cannot forge transactions, nor can they change balances without the user's signature.
4) Many different currencies might be trading on a single server, such as gold-based contracts, silver-based contracts, dollar-based contracts, etc. There could be dozens of different markets, for different combinations of asset types.
5) The same currency, with the same issuer, could be available on many different servers.
6) Basket currencies are possible.
The other big difference (coming soon) will be voting pools for bitcoin reserves stored by OT servers, to prevent unfortunate incidents where servers are hacked, or otherwise disappear with your funds. I haven't coded voting pools yet, but they will prevent incidents such as those that occurred at MtGox and MyBitcoin.
I hope this clarifies.
PS Here's an example of a user interface built using OT:
https://bitcointalk.org/index.php?topic=31032.msg399675#msg399675