As mentioned in the other topic, Mt Gox or PayPal or Dwolla are fundamentally different in that they are ONLY transaction facilitators between two other unrelated parties. They don't issue or keep the money, they only let two other people exchange it, holding USD/BTC briefly, and after the transaction is done, have no more obligations to those two people.
From what I understand, and correct me if I'm wrong, your idea would result in an entity that itself creates the BTC, holds it until a buyer comes along, sells it directly to the buyer for USD, and then maintains a long term obligation to that buyer by promising to repurchase their BTC with the same USD they sold for. Am I correct? And can you see the difference? I have to trust Mt Gox, PayPal, and Dwolla for a day or two at most. I have to trust your issuer for as long as I hold your money.
You are under no obligation to permanently park the money in the vault, and are free to withdraw it at any time (i.e. to a bank), but then the vault would have little to no stored fiat value and gain no critical mass (it would probably still have a relatively high money velocity). This is why a shared trust system is critical, otherwise you may remain a bailment service indefinitely. Not really a big deal per se, but you'd rather unpeg sooner than later I'd think. In any case, you are either trusting me (the participating trusted federated exchange servers), the blockchain, another financial service provider, or your bank. You still have to choose. You have to trust somebody, somewhere, eventually.
You want something akin to GoldMoney where they hold an unallocated amount of gold, in the aggregate, within a secured vault and then permit you to trade token quantities between GoldMoney approved accounts. The gold doesn't move much due to it's physical intrinsic nature (taking possession is logistically more difficult), so the trust level must be extremely high, in addition to the fact, they are very centralized. Nevertheless, you can withdraw physical gold specie if you want (for a substantial fee).
I do understand the implications, but they really don't have to represent a long-term relationship or obligation. Also, your money doesn't necessarily make a one-way trip into the vault (although that is preferable, by avoiding intermediary logistics). For example, vault-to-paypal, vault-to-paxum, vault-to-dwolla, vault-to-cheque, vault-to-bankwire, etc.
Notwithstanding, if you sold/traded the BCT back to the exchange pool (as opposed to somebody in the network), which effectively redeems your fiat, it would be removed from circulation by being put back in the primary account (the initial pre-mined origin block) to be recirculated at a future date, thus maintaining the 1:1 ratio as represented by total vaulted fiat reserve. The BCT coins would then be temporarily retired as it were.