So I was looking around for a means into this game's assets myself (at least the PS4 version), and I have managed a solution. Kinda.
Since asmodean's program (as mentioned above) was no go, I eventually found FreeMoteToolkit
for dealing with _body.bin + _info.psb.m
. But for a while I was still stuck, because I still needed that "key".
I find an issue thread mentions this post
... which is in Chinese, but ultimately translates to: the key is before "script/.script_info.psb.m" in eboot.bin.
And lo and behold, I find it.
I don't know if this key is going to vary between copies, or platforms, but it's a start.
My only problem at the moment is the way FreeMoteToolkit extracts the files. As-is, it extracts more
psb files. While those can be on-the-fly extracted too, it leads to an extract folder full of subfolders for each file, and it doesn't maintain the in-archive directory structure. (ie. in the case of O;N's sound_body.bin, the file mentions things like "./data/bgm/bgm28nl.wav", but does not put such file in "./data/bgm/".) This makes things incredibly unwieldy, especially for how many image assets there are.
Regardless, for those that really want to dig into this VN, using FreeMote like so:.\PsbDecompile.exe info-psb -k nx5ik27w2scm7 .\
... lets me extract the files mostly consistently. (I had to leave off -a for the image assets so it dumped them all to start, then do it again in full, though it still failed on some files anyway.)