For the sake of closing this issue:
Right here and now it seems to just be impossible.
Sony formats external (PS4) drives in a way that even if we find a way to “understand the partition” and/or mount it the data would actually still be encripted.
PS4 5.50+ USB hard drive, copying games | Nelson's log.
IF the drive could be “rawly mounted” (you know what I mean) then MAYBE the emulator could do all the work itself (or if the drive gets cloned bit by bit and “the virtual copy” gets read by the emulator).
I want to remind everybody that this was for the sake of “my brother playing the game he legally acquired at graphic settings not even the PS5pro provides”.
Too bad for Sony that “alternatives will always exist”.
Just release Bloodborne on Steam at this point…