Yep Boost is not currently available for Chromecast (see this link below on what devices are currently supported).
Difficulty to say when other devices will be added to the Boost service, because NowTV always keeps stuff in the pipeline close to their chest and never divulge anything until it's official release.
Just be aware at the moment only Sky Sports Main Event & Sky Sports Premier League play at 50fps with a Boost Pass with a compatible device.
The remaining nine Sky Sports channels are still 25fps 😠
Hopefully the other nine sports channels will be bumped up with the 50fps frame rate sooner rather than later.