Just in case anyone still wants to add outfits (in particular the Medjay assassin's creed ones), I couldn't fix olemagne's script but here's a workaround using cheat engine's scan for values:
1) Open cheat engine and attach ffxv's process
2) Load your save (doesn't matter if you load your save first or attach the process first)
3) Do an exact value search for 4 bytes for your desired outfit code in this post on page 46 of this thread viewtopic.php?p=37963#p37963
. The outfit codes are in decimals so don't tick the hex box.
4) There should be a few results - to identify the correct address, you need to right click the result and choose "Browse this memory region". Then observe the hex values - the next outfit code should appear 136 bytes later (you can click and drag in the memory viewer to count the number of bytes. Then click the first byte and look at the integer value in the bottom bar to get the value in decimals)
5) Once you've found the correct address, the byte that controls whether you have the outift or not is 20 bytes (or 0x14) from the initial address. You can just use a calculator to add 14 in hex to the initial address to find this control byte, or manually count 20 bytes (if the initial address is byte 0, you want byte 20). If you don't have the outfit, this control byte value should be 00. Change it to 01 to activate it.
6) In game, open the Gear menu and navigate to the character's outfits - you should see a new blank space corresponding to the outfit you just added. Don't worry about the blank space or any missing body parts for now. Select that, and save.
7) Reload your save and everything should become normal with the new outfit added.