I don't think it's just a matter of "unlocking" it. It seems like they just have a set number of hairstyle slots per character design and then hairstyle models were assigned to those slots, rather than each style being an independent thing that is locked or unlocked for each character.

The later slots all have the same hairstyle in the spots for the corresponding race, so it seems like one style accessed by everyone, but they're still unique per character - most obviously on Miqo'te where the ears are programmed as part of the hairstyle and some pieces need to be modified.