i tried both mateo's tutorial and bruteforce method. neither worked. I'd recommend just updating to 4.30cfw (rebug/Rogero). It's not complicated and takes 20 mins.
Just as a warning though, for those playing the duplex release, you're gonna have freezing issues, especially if you are running it internally. It's something to do with the game's audio running too fast from the HD instead of it running at a slower BD speed. Makes it go out of sync and freeze. I'd recommend waiting for the PSN version as that is made to run off the HD.