In my original "holy grail" post, our solution was to use issuers in combination with colored coins (and we will still probably support that)
Please, do! Many people (including me) count on the integration of colored coins in OT.
As we've been developing the concept, it's become apparent that we need to solve a more fundamental problem first: how to achieve contract recourse and dispute resolution in a decentralized environment.
You probably can't. I'm afraid it isn't possible to have a truly decentralized full-fledged exchange for centralized assets like the dollar or the euro. You always end up with some sort of escrow agent with a distributed security like multisig rules to be the custodian for the "real" stuff. You can always use smart contracts but that suggests you'll use parallel investigative, judicial, and law enforcement private agencies. Nevertheless, I like your concept for the distributed Bank. This certainly has a future. I'm not sure if it is an alternative to Ripple gateway or is complementary.