The Chromecast Bridge Plugin is designed to use Chromacast players with the Squeezebox Server. In this way, they become Squeezelite players, so to speak. However, it is not possible to turn Max2Play into a Chromecast player in this way.
I am also not aware of any other way to support Chromecast streams directly. As Heiner said, you can use DLNA apps to stream to the Sevrer. For Spotify, you can also use our Spotify Connect plugin or Spotty for the Squeezebox server. To use other sources such as YouTube, a connection via Bluetooth is probably the best and easiest choice.