I'm currently writing a small converter utility to convert PLS/M3U/etc playlists to UPL and back, and to serve as a reference implementation/proof of concept. If you want to follow development, feel free to star this repo:
https://gitlab.com/universal-playlist/pls2upl
You can also star the repo of the spec website itself:
https://gitlab.com/universal-playlist/universal-playlist-spe...