[EDIT] Adding this is a lot uglier than I expected, thanks to (a) songslots needing to be sequential, (b) a certain someone leaving the completely inappropriate line
resetContinuity() in the script for the index node, and (c) the song I chose being too similar to the Energy Form song. I've updated the instructions in this post, and you can download an archive of the files I've modified/added
here.
Although actually, Edwards said he knows how to add his own songs to mods. Maybe he'd know how to make one that teleports you to a different map (ie. the index).
Hmm.. I can't find the files that tell the main game what each of the songs does so I wouldn't know how to do it. >.< All I can find are things relating to the 'slots' each song is in and stuff.
Wait- that information isn't well-known? I could've sworn I found out how based on something here on the forums, although it might have been an off-hand comment by Alec. Anywho, here's a quick bit of code for a song to send you to the index map- I've been using something similar in my own mod-in-progress, as it's a lot easier to just sing a few notes than to remember the names of all your maps.:
Place in a file "songs.xml" in the scripts folder:
<Song idx="100" name="Go to Map" slot="12" notes="7 4 6 5 3 " script="1" vox=""/>Place in a file "songs.lua" in the scripts folder:
function castSong(songIdx)
if songIdx == 100 then
loadMap("index")
end
endOpen up the script "node_index.lua" and comment out the line
resetContinuity() near the beginning of the file (or possibly just add
learnSong(100) after it).
And just to be thorough, add the following image ("songslot-12.png") to the graphics/song folder of the mod:
[EDIT] Forgot to mention that you'll also need to add the line
learnSong(100) to mod-init.lua, and the notes you need to sing are:

Also, you'll lose the song if you ever visit the title screen map, due to its use of resetContinuity().
- Edwards