The attack in Thailand wouldn't have been possible without irresponsible lending by banks in risky real-estate speculations (where have I heard that story before)?
And how about when the Bank of England lost to a speculative attack by George Soros, in which he profited some 1.5b? http://en.wikipedia.org/wiki/Black_Wednesday
What makes you think people are not going to irresponsibly lend GoldCoins?
I guess it doesn't really matter, since no one is going to buy the GoldCoins in the first place. Faith in this escrow mechanism is not going to exist.
It is absolutely true that a central bank cannot keep a fixed exchange rate while also manipulating interest rates in a free market economy. I would never argue against you on that. However, MasterCoin makes no attempt to control interest rates, therefore there is no competing goal to allow a speculative attack.