The full list of all the unlocked songs is a different story lol. I've made it so that, in the latest version of the script, any songs in your game's Audio/BGM folder are automatically marked as unlockables, whereas RTP songs will only be unlockable when specified. That's got to do with ease of setup for the user. Since most users that seriously want to create a good game problably don't use too many RTP songs, I figure it'd suck to have them all loaded into the script standard.
To see a full list of the songs I've actually put in the script as a standard example, you should find the config section in the script and change "UNLOCKABLES" to false. That way all the songs I've specified are automatically unlocked for use.