Sorry, I hit the button too early...Isn't this product similar to what you're talking about? WisdomTree CBOE S&P 500 PutWrite Strategy Fund (PUTW)?
"PUTW invests in one- and three-month Treasury Bills and sells or “writes” S&P 500 Index put options. The number of put options sold is chosen to ensure full collateralization, meaning the total value of the Treasury account must be equal to the maximum possible loss from the final settlement of the put options at expiration."