Chromecast used to support streaming from the casting device, i make an app that used to use it. Sadly they no longer support that feature. Having everything on your device allows considerable more freedom.

It's definitely still possible since effectively the cast device just runs a (display-less, in case of speakers) browser instance navigating to some URL and communicating with the remote using a message passing API, right? You can display whatever you want in that URL, including HTML media elements playing back HLS or MPEG-DASH streams.

I believe they used to have some default receiver app implementations that you could just send a media URL and they'd play that back; is that what they removed?