Sadly this is out of scope for the Plex app as it would require the underlying Android system to handle support or for us to recreate every screen in our app to support such a scenario.
Unsure on the Google TV but on the SHIELD in the developer options theres an option for 21:9 resolutions I believe as long the device is emitting these. You might also be able to get support by adjusting the resolution of your device via ADB. See; How can I set a custom screen resolution for my Android TV box? - Android Enthusiasts Stack Exchange
As for HTPC, there is lots of remotes you can get that will act like an Android TV remote either via bluetooth or with an IR sensor on your PC.
