Sadly it isn’t. Chromecast communication is only in lan. Unless you basically expose every chromecast outside your lan, you can only communicate with them in your home network. Sure somehow you can hack that together but it would be really insecure and not good solution.
Your only hope is 1: ST allows more code execution on the hub (I don’t see that happening, who know if the hub apis will be around a year from now) 2: Google might allow cloud control of chromecast devices.
Personally I believe google is hard at work on that. They added the ability to launch receivers and media playback from the cloud already, sadly not public, internal google only. Having full playback control from the cloud would be the next logical step. Check out the new Google Home app, it is basically like ST. And in all honesty as soon as the Google’s Home app is a decent smart home platform, I’ll be leaving ST.
The complete lack of any communication about new hub apis is just unacceptable. Googles Smarthome stuff is better documented then ST right now… or what feels like over a year now.