No, not really. I've actually had similar issues with an answer in the past - [I ended up posting a low bounty, then promising a higher bounty once answered][1]. Obviously its on a honour system, and you're going to have to follow up (and It was a special case) but thats one way to handle it. 


  [1]: http://meta.superuser.com/questions/3742/can-i-promise-a-bounty-but-only-put-it-in-place-once-there-is-an-acceptable-ans