Support Libreelec By Creating a Kodi Addon

[Saroufim]

It would be nice if the protocol can be embedded into Kodi via an addon to support libreelec.

[Rafostar]

If you just want to play the file selected from cast-to-tv gnome extension, then you can set your receiver type to Web browser | Media player in extension prefs, select the desired video like always and in your Kodi box play it using internet streams (video url). The url is always http://YOUR_IP:4000/cast.
This should allow you to play media in Kodi without playback controls from your PC (only control playback from Kodi box).

If you want to make the cast automatic (without selecting predefined url each time) with remote playback controls from gnome-shell, then like you suggested a Kodi add-on would be required, but I lack the knowledge how Kodi add-ons work (seems like they must be done in Python).