Sure, the short answer is that I don't know. I'm asking questions for the sake of being able to provide as much information as I can. For other playlist formats, the playlist doesn't contain metadata at all, but XSPF seems to be an exception so I was trying to find out more about how XSPF functions for you. My expectation is that whatever MusicBee outputs for XSPF wouldn't include custom tags if it includes tags at all. It's definitely not configurable from within MusicBee.