Perhaps Goldman would be kind enough to sell you some puts: http://blogs.wsj.com/deals/2011/01/14/struck-out-on-facebook...

Create a contract that allows you to do the following:

1. Buy Shares for $x from person A

2. Immediately share sells of Facebook

3. If the price of facebook drops below $x, rebuy for $y

4. Transfer new shares to person A

The tricky part is #3 on what conditions do you have to rebuy shares when $y > %x.

Convince a banker to write an over-the-counter put option on private equity? Though the banker will presumably want a really low strike price to justify the risk at any premium.

put options