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.
According to this recent updated article, HD Boost is now available for Chromecast Generation 3 devices and Chromecast Ultra (unfortunately it looks like the original Chromecast and Chromecast 2 are not Boost supported 😡).
And so I did it, a shiny (well, matt) new Chromecast Gen 3 and £6 into Now TV's coffers for three months of HD Boost on offer.
Desperate times so I watched some live golf and football from years afar with Eric Cantona.
Lovely, 50fps makes such an improvement, so I thought I'd post this following a sense of behoovement.