But what I am proposing isn't just an abstract idea. I'm saying we need to actually start making a security using an idea like I proposed or maybe just following the colored coins proposal.
OpenSource software exists right now running on servers and being available for listing and trading assets denominated in BTC using Open Transactions (via blinded tokens), Colored Bitcoins and Ripple. Also there are centralized (often closed source) exchanges that put a strong focus on crypto (e.g. MPOE where every trade etc. is signed with PGP keys so it is externally verifiable and auditable).
In contrast your idea, even though a nice one and probably relatively easy to implement, is NOT implemented anywhere that I know of, even in a test/beta scenario.
The problem with these decentralized exchanges is that there are few incentives to operate one. Ripple solved it by creating XRP and they get a lot of hate because of that. Colored Coins seems to piggyback as much as possible on Bitcoin, so it wouldn't really need much anyways besides a note on the company website to trade this asset and OpenTransactions... well I have yet to see someone even using them, though the software exist for years now.
We need people starting a security on ANY existing platform and stop about designing ideas on how to come up with yet another decentralized trading platform. The platforms exist for months and years already, it is just that issuers of securities seem to be more happy with using centralized websites than not-so-nice but secure and distributed exchanges. Similar issue like Chaumian blinded money (which was even being tested live) compared to PayPal (or bitcoin-qt vs. inputs.io). Their selling point is simply "trust us, then all you need is username + pwd". People care about simple more than about security and privacy, even(!) in the Bitcoin ecosystem!