In order for the peg to work there must be some sort of mechanism to pump the price below par and debase above. I think for it to work the Blockchain would need access to all market data and have the ability to print or buy as much SBD needed to restore the peg. Ultimately SBD might need to be delisted from every exchange beyond the Blockchain’s steem/sbd pair. I don’t think that is possible so we may never see the usd peg work properly.