Pretty sure this is what you're looking for. It's a plugin for Pulseaudio that adds arbitrary DNLA/Chromecast devices.

http://www.webupd8.org/2016/03/how-to-stream-audio-to-chrome...

Edit: And if you use Wine with Winamp, you can then steer Winamp to use chromecast as well. And as an aside, I also used Wine to make Windows programs (not the desktop) remote-run by running ssh crankylinuxuser@server -X -f "wine winamp.exe". And with Pulseaudio, redirection is trivial.

No. Chromecast isn't free software or open protocol. Official support only comes through google's closed sdk and libs. Some projects like vlc have reverse engineered it sufficiently to support it somewhat, but it's far from perfect.

Yes there is: https://github.com/masmu/pulseaudio-dlna

I think there's a cross-platform chromecast library, but you need a developer key as part of your build/dist to use it. I seem to recall playing with a node module a couple years ago, and it wasn't THAT complex.

Integration on Android proper is probably going to be easier.