we can borrow BitUSD from wallet, but don't use these funds to buy BTS or bitBTC.
we can sell it for fiat USD, or other smart asset with low risk, such as bitGold, bigCNY, bitEUR ....
we should use these funds to support gateway, support liquility.
if too much people borrow BitUSD to buy BTS, it will  hurt the peg, just like now.

people who without short position can buy BTS with bitUSD.
or you can prepare enough fiat USD as reserve for your short position.