I think Open Transactions may be a hell of an overkill for this.
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I believe that #assets-otc or #otc-assets project does pretty much what you describe in your original specs minus any automated trading capacity ie open bid/ask.
I would much prefer the flexibility of trading market functionality over an OTC functionality.
The colored Bitcoin idea sounds interesting, as long as it exists alongside Bitcoin, not as a part of it, but, again, it may be overkill for a whole new block chain just for this.
The beauty of "colored bitcoins" is that it can exist within bitcoin without any change to the protocol (it only requires a client that can recognize the "colors" by searching the block chain) so there is absolutely no need for another block chain.
Colored coin is an interesting idea that leverages proven blockchain. IIRC, it would not require an alt-chain. But how can CC be used for open market trading and not just OTC transactions?
Decentralizing record of ownership/brokers/dealers, which is the general direction being proposed, makes an open trading market somewhat more difficult.
Re: OT. What I like about OT is that it already has P2P asset transfers + a trading market. I'm not sure how mature, robust or resistant to attack this market functionality would be.
It is certainly possible to have a single GUI for Nym management and stock market. My only doubt is that OT creates it's own set of virtual assets for trading. Those virtual assets would be backed by BTC and BTC would have to flow in/out of the OT system.
Re: Trust. What's nice about digitally signed p2p asset transfers is that parties can prove ownership in cases of malfeasance. A centralized record of ownership, in the example case being held by the asset issuer, creates an opportunity for fraud that the public would be unable to scrutinize. This wouldn't be an effective measure for wholesale fraud (as you point out would be an inherent risk in any of the approaches), but for individual cases where either party can prove ownership or payment with signatures.