okay, so i simply decided to figure out these files already.
from what i know, the cdrom.mdg file goes like this:
getbits AAA1 20 # file offset divided by 0x800
getbits AAA2 4 # file flag
getbits AAA3 8 # remaining "zero" bytes of a file
with that in mind, i decided to write a script that could "extract" these files from the cdrom.mdg file so long as ffxdemo.bin is present.
one thing to note though is that this ".mdg" file can involve the entire contents of the disc if necessary, in fact this has been the case for Chrono Cross(some two-disc game) and Dewprism/Threads of Fate(some one-disc game) whose contents were stored with a similar structure.
for now, this script can only load both ffxdemo.bin and cdrom.mdg and it's not suited for reimporting by this point so take this with a grain of salt