Hasn't that work already been done? Cyanite and KairosDB both plug in to the broader Graphite ecosystem (more or less) and use Cassandra as a data store.

Time series data has also been a particular focus in the Cassandra community. DTCS was too complicated, so they came up with the easier and faster TWCS. I don't think this is on you, but I'd love to see a comparison with the latest stable 3.x and a multiple node cluster.