I couldn't get the harmony remote to control the app and chromecast but I managed it the other way around.
Using tasker, with those 2 plugins I managed to put a shortcut on my android tablet that starts youtube tv, starts the cast and issues a command to my harmony hub.
Plugins are :
AutoInput
Tasker plugin for Harmony
I also used AutoNotification to create a notification with buttons to stop everything and the TaskKill plugin to make sure everything is dead and not streaming anymore.
Quite a bit of work but once setup it works pretty nicely.