userdata/playercorefactory.xml

<playercorefactory>
  <players>
    <player name="Photo-Reframe" type="ExternalPlayer" audio="false" video="true">
      <filename>/home/kodi/.kodi/bin/run-script.py</filename>
      <args>--play --fullscreen --read-only "{0}"</args>
      <hidexbmc>true</hidexbmc>
      <hideconsole>false</hideconsole>
      <warpcursor>none</warpcursor>
    </player>
  </players>
  <rules action="prepend">
    <rule filetypes="m3u" filename="playlist.*" player="Photo-Reframe"/>
  </rules>
</playercorefactory>

Into the kodi.log you can read that the player was actually created, but you have to enable the DEBUG logging (Settings ⇒ System ⇒ Logging ⇒ Enable debug logging), otherwise the WARNING message is rather misleading:

WARNING <general>: CPlayerCoreFactory::GetPlayer(Photo-Reframe): no such player: Photo-Reframe
  DEBUG <general>: CPlayerCoreConfig::<ctor>: created player Photo-Reframe

Context menu Play using…