I have this working for FF8 and it's quite simple (note that I use Linux here. on Windows you'll need something like DaemonTools).
For FF8, just extract the LOCAL_MACHINE\SquareSoft\Final F..\ data from registry and modify as needed.
I have my always ready ff8-install.reg:
REGEDIT4
[HKEY_LOCAL_MACHINE\Software\Square Soft, Inc\FINAL FANTASY VIII\1.00]
"AppPath"="H:\\Personal\\FF8\\Game"
"DataDrive"="f:"
"Graphics"=dword:00000000
"GraphicsGUID"=hex:00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00
"InstallOptions"=dword:000000ff
"MIDIGUID"=hex:00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00
"MidiOptions"=dword:00000000
"SoundGUID"=hex:00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00
"SoundOptions"=dword:00000000
The data drive must exist. (I use wine to create a virtual CD/DVD pointed to some folder).