Conveniently enough this required only removing decryption from my previous hack: cpk_unpack 0.7 beta 6 with ITOC backup (and no decryption unlike beta 5). As for what the HCA-headered files inside are for, your guess is as good as mine.
Though it rather looks like audio, I'm not up for hacking it just now.
is there any way to repack the CPK in the same way? I already tried apps such as CRI Packed File Maker but they don't work, probably because of the ITOC backup or sonething like that