This will be coming shortly, apparently won't be a percentage offset though but a fixed value:

https://github.com/BitShares/bitshares/issues/945

Is it possible to place bitusd buy/sell order at a relative price of the peg price? e.g. 0.99*peg price. So when the short wall is moving, the buy/sell wall is also moving accordingly.
With this feature, I will be happy to place a pair of buy/sell orders to make profit. e.g. buy at 0.98*peg price, sell at 0.99*peg price.

This trick could be done by a program which keeps placing and withdrawing orders. But in the wallet it costs a small fee to place order. The robot may soon run out of balance if peg price moves around but triggers no transactions.