I know storing coins on an exchange long term is a bad idea because Mt. Gox, however, Bittrex paid me my fork coins, BTG and I think they also paid BCH to people with BTC on their exchange prior to that event as well. So I would trust Bittrex at least if you had BTC you wanted to split, and if like you say the coin is going to develop at all the exchange will likely give that value to you, even if it is a later date, and a lower price by the time they do.