Clunky workarounds
Given that there is no support on the OS side, here is a the workaround applicable only for local/LAN and network-streamable 21:9 videos that can be opened with VLC (or kodi):
- In the monitor settings on-screen menu accessible via the physical buttons on the display select Ratio -> Wide — this will distort the UI and stretch everything to take the full display width.
- Open the video file/URL in VLC and in the three-dot menu repeatedly tap Resize until the FILL mode is selected. Given that VLC can browse LAN hosts over samba/FTP/SFTP/FTPS/NFS this can cover some but not all use cases.
Kodi can also be configured to change the video aspect ratio for videos so a similar effect can be achieved via the cogwheel click -> Video Settings -> View Mode.
With this workaround you get full-screen video with the correct aspect ratio, but horizontally over-stretched UI.