OK,
I tried to write the same tag to another mp4 file which has not been tagged with an external editor.
This time everything works as expected under MC in a correct way, but I cannot find in my Hex ediitor the field 'episode' I have written ..
If I open this file in mp3tag, it does not show the episode tag.
It seems that MC can read this tag if it exists, but does not write anything... except the XML_Carside.
I suppose that for MC this "external" custom tag is read only in this case ?