Yes, I got my old Chromecast to work with Jellyfin. Heads up: You'll have to redirect DNS from 8.8.8.8 and 8.8.4.4 to your router if you'd like to use a local hostname (i.e. in your LAN), though, otherwise the Chromecast will never use your DHCP advertised DNS service.
When I used a Chromecast, I was fond of an even more direct method. On my laptop I ran a Node CLI called "peercast" [0] with a single argument (a magnet link), and it streamed a video file while downloading its torrent, and since it called the torrent library directly, it could prioritize downloading the earlier chunks of the file so I could start streaming the video before the download completed.
It looks like that repo hasn't been updated in a while, although it might still work. It's an offshoot of peerflix [1], a Node CLI that streams video from a torrent to an h264 playlist. So without peercast I would just open my browser to the local network address where peerflix was hosting the h264 playlist, and use the Cast button to stream it to my device (which I believe technically means Chromecast "takes over" downloading the playlist, rather than my laptop pushing the video to it, so I just needed to use a URL with the LAN IP of my computer).
