I am not sure I understand exactly what you mean since I never figured out the SQP but the first thought that came reading this why bitbtc should be treated differently than bitusd. i.e if you think that there is a need for real btc to collateralise bitbtc then real usd should be needed to collaterilize bitusd as well..
The volatility of the BitBTC/BTS market is greater than that of the BitUSD/BTS market so shorts are more likely to get margin called which means they incur the cost of the SQP more often, which in turn creates a premium for BitBTC since shorts are willing to purchase BitBTC up to a 10% premium in order not to incur the cost of the SQP.
I think that the market could work just as it works with BitBTC or BitCNY markets but the devs/committee/witnesses are unwilling to lower SQP.
Also it is not possible for the Bitshares blockchain to hold fiat collateral, where as it is possible for the Bitshares blockchain to hold bitcoin collateral, because all that is necessary is the processing of information. I just don't know how feasible it is to implement.