You go long a small amount of BTC on each exchange (> your max position limit, say 3 BTC), and as long as your strategy makes more money than the carrying cost of the long position (which for any real strategy it should be paid off in days), you can arb. This is how HFTs could still market make in CAC when the uptick rule was on.
How is that not low-risk arbitrage? Good HFT strategies typically have returns of 10-20% daily on capital, there's relatively little risk holding long to allow your strategy to trade when you cover the maximum possible cost of carry in a week.
