I think you are too late to the party. One of two things would have to happen. Provided that blockchain is still going, a withdrawal will still work going to the old wallet. From there you could send them to the burn address with the correct wallet for that. Then the possibilities are:
-zeded, out of pure goodness, takes from his own coins to give to you.
-if a new client is issued there could be a hard fork for a new genesis block. I would imagine a lot of people would need to be waiting with whistle coin for this to have a chance at happening.
-you are completely out of luck. Bittrex has had the coin with warnings for what seemed like months, and then the exchange of the coin stopped. You had all that time to find out what was happening.
-any other possibilities are currently out of my scope of understanding.

I wonder how many whistle coins are still on bittrex, I haven't heard an answer here.