I'm a little confused with this. I know it will be discontinued in 2016 after Shinkansen arrives to Hokkaido but I'm not sure if it's running now or not, and if so, how reliably it's running.
JR Hokkaido has a pdf about Tokyo-Sapporo routes that mentions the Hamanasu but I'm not sure how to read that, is it running only 3 days in October ? (Can't copy paste from the pdf)
Hyperdia doesn't list it as an option. (I've even tried explicitly between Aomori and Sapporo, but then again it doesn't list Cassiopeia either)
JPRail however, says that it does and the impression I get is that it's more frequent than the JR PDF suggests. (Article dated Aug 2015)