Yobit and HitBTC have already created accounts with SBTC and HitBTC says they will credit Lightening Bitcoin as well. What is Bittrex saying about what their going to do?
Why do you need exchanges to support hard forks? Just get your Bitcoin funds to an address, the private key of which you have access to, instead of relying on an exchange. After the fork is announced at a particular block, transfer Bitcoin to a new address and use the first address' private key to obtain the forked coins and further transfer it to an exchange if you wish to trade it instantly.