You don't which is why I indicated you can NEVER decentralize the fiat half of the exchange however it wouldn't require you to deposit any bitcoin or xCoins w/ the decentralized exchange which would be an advantage over current setup. Trades could occur decentralized, xCoin balances could be held outside the exchange, you could have multiple parties involved in one decentralized cloud however .... the conversion of fiat to token WILL ALWAYS require a trusted 3rd party.
Simply put the linkage between fiat world and crypto world will always require a trusted 3rd party however using a decentralized exchange limits the control and assets that trusted 3rd party has access to.
For example say you want to buy Bitcoins. You would find a moneychanger you trust. Via some merchanism you purchase USD tokens for USD. 1 USD token : 1 USD. If you want Bitcoins or xCoins, or abcCoins you find someone within the decentralized exchange who is willing to trade USD tokens for Bitcoins. The trade occurs they gain ownership of USD tokens (likely w/ intent to exchange those for meat space money) and you gain BTC.
I'm not clear of the utility of this procedure? Who is this USDCoin issuer? Is it an exchange? How is this USDCoin different than Dwolla and MtGox? How do we know they will buy back their USDCoins when the person stuck with them in the trade wants to cash them in?
The USD "token" is simply a digital manifestation of a promise by the money changer to payout USD on demand. If the money changer can't be trusted then it is worthless and you lose the value. That isn't any worse than current system but it does limit the assets under risk.
Everything can be decentralized except the linkage to the meatspace world so the goal would be to keep that linkage as small as possible.Current system (Mt Gox or other centralized exchanges)
Bitcoin balances: held by exchange (trusted 3rd party)
Trades: controlled by exchange (trusted 3rd party)
USD balances: held by exchange (trusted 3rd party)
Decentralized Model
Bitcoin balances: held by individual
Trades: occurs within decentralized exchange
USD Balances: held by trusted 3rd party (in form of promise to redeem USD tokens & actual USD balances held)
In current model if your exchange can't be trusted they can steal from you three ways
a) steal your Bitcoins
b) steal your wealth via manipulating trades
c) steal your Fiat money
In decentralized model if your moneychanger can't be trusted they can steal from you but only one way (and only part of your wealth)
a) steal your fiat money (by issuing tokens they never intend or can't redeem back for fiat money)
It isn't perfectly decentralized but like I said you can't decentralize fiat money which is inherently centralized the goal would be to limit the exposure of centralized control. The advantage is the same decentralized cloud exchange could allow trades between various tokens (USD, Gold, Silver, EURO, S&P 500 shares, etc) while limiting the amount of assets held by the exchange to just the "meatpsace" linkage.