[ashlar]

xbmc.executebuiltin('PlayMedia(plugin://plugin.video.youtube/play/?video_id='+ url +')')

This is what I am using in Python.
If you want to use it from a skin command then it's should look like that:

PlayMedia(plugin://plugin.video.youtube/play/?video_id=WFwHq6cY040)

Thanks for your answer. May I be so bold as to write to you privately about this? I don't want to pollute the thread with a matter not relevant to the grand majority of users.