So I tried sideloading the WiiM Home app on my Chromecast with Google TV. The results are.. well, interesting ¯\_(ツ)_/¯
It only shows in portrait orientation and navigation doesn't work with the remote buttons so you need to connect a mouse/touchpad to show a cursor & emulate touch input. Weirdly enough, the (custom) launcher is still shown behind the app.
Far from ideal, but maybe an idea for the future
@WiiM Team to have an Android TV compatible version of the app - for those who like to show a player with album art on the big screen.