But to be useful at least for me, it'll need to both accept streaming inputs (i.e. be a client for Google Play or Spotify) and to stream its output to Chromecast.
It's a different world now.
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.
Integration on Android proper is probably going to be easier.