You can also create these .pla playlists yourself. Apparently you just need an empty file “playlistname.pla”, and a text file called “playlistname.pla.refs” that contains one path per line, with the prefix /mmc:0: for the internal memory or /mmc:1: for the memory card. For example
/mmc:0:/MUSIC/somesong.mp3
/mmc:1:/MUSIC/someothersong.mp3
Backslashes work too for the rest of the path, just the first character needs to be a slash. And the file needs DOS line breaks, UNIX line breaks don’t work.