If I remember rightly this was a plan they had very early on in ARR's development but seemed to have quietly abandoned at some point prior to ARR's launch, possibly due to the sheer complexity of it and so only the company chocobo ended up having customization available. Given the large amount of different mounts available even at ARR's launch, this is not surprising, as it would have just been far too time-and-resource consuming to get to work, so the plans were dropped.

Which is unfortunate, as I would have liked to have been able to give my magitek armour a new paint scheme at least.