I think only adding glamour items and hairstyles themselves could be problematic:

-Copyright
-Existing items modded in
-Unlockable hairstyles modded in
-Dye variations

Some of the existing items require running a dungeon, trial or raid, getting an achievement, purchasing from the MogStation, completing a quest, crafting, and so on. Unlocking a hairstyle I think is either an item in game or MogStation. I think most of everything else can be fine. As for the textures of glamour and hairstyles, I think that would be possible w/o it being a back door for people to use it to mod in items and hairstyles they cannot otherwise use but exist already in game, but I'm no modder.