What is Reddit's opinion of SignagePlayer Android (FREE)?

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if this is the app you're talking about, no. it hasn't been flagged for android tv. that doesn't mean it can't be sideloaded and work, but i don't have time to check that for you.

Raspberry Pi with the videos stored on it. Build a webpage running on the Pi that will initiate the Pi playing a video through omxplayer or similar and using PHP to trigger the commands.

Then install a kiosk app on the Android tablet to keep it on your custom web page. They click a thumbnail, a video starts playing on the screens attached to the HDMI output of the Pi.

https://www.raspberrypi.org/documentation/usage/video/

Edit: There's also this: https://play.google.com/store/apps/details?id=air.me.signage.player