Well, that was quick... someone on GameFAQs extracted all the cheat codes from the game and is going to post all of them in a couple hours. He's still trying to work out what everything does. Most of the first batch he posted just start you with a relic and give you infinite use of them except this one.
Code #43: STGZURSC (starts you on NG+ with everything unlocked, 0 music sheets collected)
aww man.
Although I do want to play around with all the cheat codes. I kind of wish the people that kickstarted $100+ had a chance for their codes sent to them to be their very own secret, at least for just a bit.
Code 42:
RQQYUKIJ Starts New Game + with no upgrade.
^ Perfect for the people that want more of a challenge right from the first play through.
Edit:
I'm not sure if that Gamefaqs list of 321 cheat codes is ALL of the codes,
The 3 codes we knew about already had "&" and "!" characters in them and numbers "1" "4" and "2", the gamefaqs code list looks like it's just alphabet based with no extra characters
I'm thinking there are probably more codes, if somebody brute forces it with all the punctuation and numbers not just the alphabet.