Recently found Rock Band Unplugged again and have been playing it on emulator. I was recently given access to the alpha build of the game from late 2008, back before the game was called Rock Band Unplugged and was just called Rock Band 2.
Upon digging around in the files and looking at the setlist in game, there appears to be a massive amount of songs that don't appear in the final game, including Almost Easy (A7X), Panic Attack (Dream Theater) and a few other early versions of final charts.
There are also a number of songs referenced within strings of text contained within the game's chart files (.str files) which do not appear in any version of the game. This includes a number of Green Day songs (boulevard = Boulevard of Broken Dreams, brainstew = Brain Stew, american = American Idiot, Whatsername) and some Rock Band 1 songs (ballroomblitz = Ballroom Blitz, entersandman = Enter Sandman)
The way that these songs were referenced interested me. They were all linked by a file path. "songs/(songname)/songname .mid
The songs take midi files for the chart but they're obviously encrypted for use on the PSP. These are coupled with the audio files (.ATM) which are multitrack audio containers that have multiple audio files inside of them, containing each of the 5 stems used in gameplay.
ALSO within the game files are multiple references to file extensions used to make regular RB2 xbox CON files such as multiple Xbox compatible image formats and .dtb, .mid + .mogg files
If we can somehow figure out how these files are encrypted and possibly use data from an already existing CON file, we could be on our way to RBU customs.
People have touched the subject before, but with the current knowledge of the community and the recent discovery of Guitar Hero On Tour customs AND how supported the PSP modding community is, I think we have a good shot at this.
If you're interested in helping me a couple of others on our journey, send me a DM either here or on Discord at SanicStudios#2512