Unfortunately, google chromecast is a special kind of media player. IMHO it’s designed to play media mirrored by android apps and does not support playing “local” stuff out of the box. My mpd media player can access playlists located somewhere and plays items from it.
I had a chromecast myself, but it was too restricted for my needs at all.
I am not sure, but https://github.com/balloob/pychromecast/blob/master/pychromecast/controllers/media.py or https://github.com/balloob/pychromecast/tree/master/examples gives some information. But there are only google things like youtube or single mp3s as input.
The hard way to go could be via plex which can cast to chromecast as well. Plex could be automated via hass again. 
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