This is the hardest part for many people to understand until they see it in action. The reality is that every market player is constantly adjusting their assessment of the value of BitUSD based upon their observations of USD / BTS. The thing that ties them together is market consensus and this is a concept that takes a degree of intuition to understand.
Your suggesting is a 'price feed' from a trusted source to be used to settle positions and is what all of our competitors feel is necessary. But if our hypothesis is correct these feeds are unnecessary.
If we rely on people's believes or confidence , then a sudden flash crash kind of accident will break the belief/confidence and hard to recover. Take a look at all the derivatives, futures and options will have the expected trend going along with the underlying's price because they are settled/executed based on underlying's price, index derivatives are settled with cash rather than index(which is only a concept rather than a physically existed equity) and it works fine because they are settled by "FEED PRICE OF UNDERLYING", it is not only about margin and collateral, it is also about how to finish the strike/execution and settlement. I still think that a "realUSD price feed" from a trusted exchange with suitable volume will be a stronger and more solid foundation for the PEG mechanism. With this simple linkage from realUSD and bitUSD, it will give arbitraging people a guarantee that they will have their profit realized once they do it right, instead of praying for all market participants to believe in the same thing they do.