Intersango already does this.
Intersango just reduces the amount they charge liquidity providers and increase the commission that liquidity takers pay. At Intersango, right now
Liquidity Providers PAY 0.35%
Liquidity Takers PAY 0.95%
True enough. I was sloppy there. You're absolutely right: it's not possible to get a net reward from an Intersango trade. It's just varying degrees of cost. Unfortunately.