I believe it can be done using derivatives.
Then you don't understand the problem.
Fiat liquidity in a P2P exchange means that I can take my digital representation of dollars and reliably and consistently turn those into a physical $100 bill in my hands to put gas in my car and buy groceries.
Solving this problem means coordinating actions in meatspace; a complete solution requires very little coding and a lot of HR.
No, you are confused. I am located here in Singapore.
The Brunei dollar is pegged to the Singapore dollar, but printed by the gov't of Brunei. Maintaining this peg requires matching Singapore's monetary policy, so that you receive the same interest regardless of whether you deposit your money in a Brunei bank or a Singapore bank. (or likewise pay the same interest rate regardless of whether you borrow from a Brunei bank or a Singapore bank). It also requires maintaining a stable fiscal policy, since otherwise the Brunei gov't will turn to the printing press and this lowers interest rates.
If Brunei offers a higher interest rate, then money will flow into Brunei until paying back the accumulated inflows becomes unsustainable. At this point, everyone with money in Brunei will suddenly race for the exits, causing a sudden collapse. This is what happened with Pirate. This is also what happened in Cyprus (8% Euro interest for some sketchy Russians) and extremely high risk lending and speculation to support it. Extremely unstable, yet a worthwhile risk for Cyprus because a) they are booger-sized nation and can gain a lot from international capital inflows (much like Singapore) and b) expect a bailout from the Eurozone (unlike Singapore).
First and foremost, the system depends on a flexible interest rate policy. This can be implemented by adjusting the interest rate itself or adjusting the money supply.
As far as I know, no one involved in Mastercoin understands this. Until they do, all attempts to set up MastercoinUSD will ultimately fail. The key thing is that they will fail from ignorance not because it is actually impossible (see Brunei). They will also most likely fail through sudden collapse rather than through a gradual process.