I just made a playlist in Rune and found where it is stored, which is at /var/lib/mpd/playlists in m3u format. For my songs it has this structure
USB/SAMSUNG256/1970s/Abba - Knowing Me, Knowing You [mp3clan].mp3
USB/SAMSUNG256/1970s/Boney M. - Brown Girl In The Ring [mp3clan].mp3
Take notion that 1970s is a folder.
Does this ring a bell with you? However, making playlist is rather simple and works very fast in Rune. Click Queue and click Save playlist (left side bottom)
To get at /var/lib/mpd/playlists use WinSCP software
https://winscp.net/eng/download.php Elsewhere in this forum you can find out which information to fill in and connect to Rune.