This would essentially be a bitcoin loan - I've never heard of such a thing, but i don't follow them very much (I do work in finance and trading though).
Literally, what is the bitcoin interest rate?
Is there even enough volume ooh these lower tier exchanges to support shorting the spot on them?
EDIT: i didn't find the interest rates but i did see that Bitfinex offers the ETH-BTC cross which is good so somebody can bet the relative value. I still think a futures market weighs be really useful for hedging purposes and reduced shorting fees and slippage.